Bob Aller (Stony Brook Univ) – Shoreward transport in deltaic systems: the coastal vacuum and reactive particle processing
Leaky deltas Workshop series
May 30, 2024
you could go to the menty poll we like to do this at the beginning of of all of our webinars to get a sense of the the makeup of our community that we’re trying to build here um this is the first question and just trying to get a sense of the career stage of our participants the next question is more fun [Music] do you see [Music] waiting for those of for those of you just joining there’s a menty poll you can join using the code listed there or if you’re on a smartphone you can scan the QR code e we’ve got eight responses but we’ve got 14 participants so can anybody who hasn’t yet please fill in the menty I don’t see where to fill it in actually where do you fill it in oh you just join so you go to mentee.com on your in your browser and then just enter that code and it’ll bring up this poll and you can submit your answers through the poll so Heather with this new setup will we be able will people be able to unmute themselves at the end to ask questions or um would you need people to um ask questions in the chat we might have to use the chat this is that set up as a webinar instead of a meeting so I’m not certain what participant uh what participant functions are enabled for this they might be able to unmute or raise hand and then we can unmute them um oh yeah okay I see if they we can allow someone to talk yeah okay we really had to button it down after an unfortunate I heard it was awful yeah okay I am GNA move to the next question because it’s a fun one the next question is please tell us what deltaic systems you are working in or how worked in and I apologize I’m not sure if I set this up for um to enter multiple responses so if it only lets you enter one just tell us what one you worked in most recently go Florida pretty good Global coverage here oh this is cool looking good everybody wow for I mean we have probably 25 20 to 25 participants but that’s pretty good uh a global span there for not a huge group of people all right well I will leave this up while um Julia gives the introduction and then we can switch it over to Bob so he can give his fantastic talk thanks everybody okay can can you hear me MH okay great well welcome everyone to the Leaky Delta seminar I hope everyone is having a good morning or evening or um whatever part of the day you’re in um wherever you are in the globe and so today I’d like to welcome um Bob Aller uh he’s a distinguished professor at Stonybrook University’s School of marine atmospheric sciences um he got his PhD at y University um and since has gone on to have a very distinguished career um he works on early diagenetic reactions in Elemental cycling involving the decomposition of organic matter dissolution of inorganic biogenic debris and the mobilization and reprecipitation of metals sensitive to um redo reactions uh Bob has worked in a variety of coastal and Deep Sea Marine areas including Long Island Sound French Guinea um and Papa New Guinea and with that I will turn it over to Bob who I think is going to share his screen um and thank you everyone for bearing with us while we um work with the new tech setup um and um but we’ll try to we’ll unmute people at the we think we can unmute people at the end to ask questions um or you can put questions in the chat at the end and I’ll keep an eye on the chat during the presentation and so now I’ll actually turn it over to Bob who will be talking about shw transport and deltaic systems uh the Coastal Vacuum and reactive particle processing okay I’m trying to change my view here a little bit no rush H oh my okay um can you see my screen okay Julia yeah I can see your screen great um I have problem all the participants too that’s what I’m seeing Oh I um alth I see all the participants on the right hand side but your screen is the main thing that I see okay super I can I can go because it’s just an annoyance to me and not really critical um so I’m gonna I’m going to talk about U Coastal systems associated with deltaic uh regions of the ocean and in particular I want to emphasize the exchange of material with the U with the coastal ocean and U outer ocean regions and something that I referred to in this particular title as a coastal uh vacuum so let’s see here we go what is going on okay so I first want to acknowledge all the uh different people that I’ve uh worked with on this and uh UPF front indicate that there’s lot lots more people than myself involved with everything I’ve talked about and uh of course like many of us I’m highly dependent on the participation of all of them and the interactions with all of them now um one of the things I wanted to start with was to say that the way we address a lot of deltaic uh problems and the interactions of Deltas with the oceans is to view them often as a unidirectional transfer of material from the continents to the oceans and it’s sort of built into the conceptualization of inshore patterns to Offshore patterns and we get uh uh we have let’s see if I can get a laser here we have arrows like this which are almost inevitably uh uni directional indicating material transfer from one region to another uh along a progressive uh pattern uh going from the land into the uh interior ocean and I’m going to talk about two such areas today the Amazon and an area in Papa New Guinea uh which you’ll see a bit later called the Gulf of Papua and I’ll explain these different um areas and regions and what they represent the numbers on these um Maps uh if you aren’t familiar with them refer to the metric millions of metc tons of sediment introduced from the different drainage basins shown and these are in fact contrastic uh drain contrasting drainage basins low yield uh regions like um uh the low gradient regions like the Amazon and high gradient regions which I’ll talk about later uh like Papa New Guinea now along the the sort of uh idea I’ve been talking about is one of the ways we look at these kinds of processes associated with deltas and inshore offshore patterns is to look at patterns like this as as a function of bimetric depth here and um in this particular case it shows net sediment accumulation rate as a function of bimmetry here and moving from the shoreline into the deep C and here is a a logarithmic diagram that you can see and there’s a few Deltas plotted here and you can see immediately that the Deltas are uh have the highest sediment accumulation rates and they’re also very very variable which uh is extremely important as I’ll talk about a bit later but the main idea is that we’re sort of it’s ingrained in the way we view things that we’re looking at Material moving from this region progressively in this region and what I want to emphasize today is these areas these bound areas that Deltas represent are highly interactive with the ocean and there’s a material transfer in both directions not just from the land to the sea but also from the sea into the Delta Systems and uh one part of this uh transfer and and uh movement of material is shown here this is a a plot from Paul Leo uh and and others showing that many large river systems introduce material to the Sea a large uh quantity of that material accumulates immediately proximal to the river mouth but also variable amounts of material Move Along Shore so we need to view these these deltaic systems immediately as actually at least uh two dimensional systems along the coastline as well as at the proximal Reg of the Delta since it’s quite typical to get 20 30 40% or more of the material that Rivers deliver to the ocean actually move along the coastline and they don’t accumulate in what we might uh refer to as the Delta proper region that’s extremely important to uh realize so I’m going to move now to talk about the uh Amazon system which is one of my favorite systems I’ve worked on it uh many de ades and I still work on parts of the system that are down drift of the Amazon this is a a the satellite real color satellite image of the Amazon mouth which is at the equator here which runs right along here here’s the Atlantic Ocean and what you can see is H this sort of classic turbid plume emerging from the river mouth moving into the ocean and then a boundary just offshore where the turbidity begins to drop of uh green water showing the incredibly High productivity that’s associated with the input of uh the river material so one one of the things we know about these systems is of course they promote a lot of of cross shelf transport of nutrients from the river here’s the Amazon mouth again this is a seawifs u satellite image showing the annual average primary production associated with the Amazon River uh inputs and Delta system here to the ocean you can see that it’s not just right off the river but rather all along the coastline uh there’s an enhancement of productivity and then there’s a a what’s called a retroflection of the input of the Amazon plume into the interior Atlantic Ocean but the my point here is that we’re sort of we’re it’s ingrain that we’re thinking of these systems as uh movement of material from the river into the ocean but if you if you look more closely at some of these patterns of for example chlorophyll associated with the river mouth here’s the Amazon River mouth here here’s that high turbidity water which is pretty obvious here here’s the green water showing you the very high productivity associated with this transition from high tur turbidity to lower turbidity water and so there’s extensive transport uh that’s obvious but you can see here these stringers of chlorophyll a rich water that are coming inshore so we can always already see on the surface of these images that there’s a lot of shoreward transport of uh of organic material produced in the ocean back toward uh the coast line so I I want to now talk more about um the different characteristics of cross shelf exchange and uh not just the seaword transport which I’ve been referring to uh here by the seawifs uh type of satellite photo but also the shw transport which I was just showing an example of in terms of the Eddie of chlorophyll a rich Marine derived organic matter shoreward for the river and I want to talk about some of the tools that we uh use and that illustrate some of the these transport phenomena particularly the shore word components of transport and I want to emphasize for the rest of the talk radiochemical tracers in particular lead 210 and thorium 234 and also a a cosmogenic radio nuclei that shley Ramen has worked a lot on and uh Ed an example there as well of showing the shoreward and a long short uh entrainment of reactive biogenic debris to the coastline and I’ll give a couple examples of the seaword transport too just to convince you that it that I’m I’m I’m aware of that as well and that that’s extremely important but one of the things I want to get across here today is that if we want to understand carbon cycling in these Coastal systems in these deltaic systems we need to view them in just that way as systems that are associated with both with Exchange in both directions and there are a range of tools not just carbon in and of itself but there are a range of tools that we have available to us to quantify these exchange processes so I’m going to go to um a diagram here which shows again the Amazon River mouth but now the thickness of sediment uh that’s accumulating off the Amazon River mouth that’s characterized by extensive remobilization and reworking by the currents uh that occur on the Shelf including tides and wind driven uh wave disturbance for the bottom and these this is the thickness Contour thickness of the lead 210 reworked layer that’s seen off this River mouth uh here uh in over extensive areas and now Steve Keel uh measured the thickness of this sediment mix layer or lead 210 reworked layer um extensively across the shelf and it’s defined by this kind of homogenization of the lead 210 profile here’s the activity of lead 210 in the sediment and DPM per gram and here is depth in centim uh depth that’s a a characteristic profile that you might find for Le to 10 in this region and so this thickness shown here represents the reworked sediment as lead to10 sees it and the gradient here right at the bottom of this pattern the gradient and activity is what’s used to U Back out the sedimentation rate in all of these sites so uh you can see that uh getting the sedimentation rate in these systems of using lead 210 can be actually tricky in and of itself now uh one of the things I I I had mentioned this earlier when uh um Kristoff was giving his talk about the rone Delta and Lead 210 is sometimes these lead to10 profiles there’s information in it that people in them that people just don’t uh look at very often they they ignore them they look only at a profile a vertical profile like I was just showing you here’s depth here here’s lead to 10 activ ity in the sediment again in DPM per gram this is at a single station E1 on the Amazon top set and you can see the thickness of this reworked layer is about 12 125 cm here and there’s a gradient at the bottom and so what Keel and others did was look at multiple sites and looked at how the variability of this reworked layer change seasonally and so there’s different times here August February May and and October and you might glance at this and see well the lead 210 mix layer is about the same all the time and so I can consider this uh system pretty stable as is but if you go in to the uh in detail here and look at the actual lead to10 activities in the sediment you’ll see they’re changing for example here you can see there are three DPM per gram at the interface here they’re 2 and a half here they’re less than two so even though this mixed layer thickness is remaining approximately the same as a function of time actually the material at this site is completely exchanging in the upper 50 cmers or so at least so this material uh is is moving back and forth from one place to another so it’s a highly dynamic system and when you look at these radiochemical tracers you need to view them them not from the standpoint of a single uh profile you need to actually sample multiple times at at sites to be U to constrain the uh sedimentary Dynamics another Tracer we use that actually also reflects this kind of disturbance and represents injection into the water column of benthic derived solutes is radium uh or different the different radium Isotopes and these show some beautiful data of radium in the bottom water the Amazon Delta the water overlying these kinds of sedimentary deposits I was just describing showing you that seasonally there are uh huge differences particularly in the radium 224 isotope in the bottom water which is a 3.7 day Half-Life isotope produced in the bottom and so what this is showing you is that there’s tremendous injection into the bottom water of solutes produced on the Shelf associated with this uh the sedentary Dynamics seasonally and radium is reflecting this injection process seasonally now one of the reasons I bring this up here is because asociated with this injection of radium uh here’s radium shown here is a phosphate that’s remineralized in the seabed and so the radium uh patterns seasonally here the fluxes of radium from the sediment in particular 224 the very short life Half-Life radium nucleid is uh showing you that there’s a tremendous injection and recycling of benic nutrients into the bottom Waters here on the Shelf now it’s not just injection into the bottom water it’s also movement of bottom water onto the Shelf from offshore and this is shown here again using 210 but now rather than the gradients in lead 210 or the vertical patterns of lead 210 in terms of activity uh you can use inventory the total amount of lead 210 excess lead 210 in the sediments and this is represents some really fantastic work uh headed by Dave demaster many years ago showing that on in these systems in these deltaic systems like the Amazon uh you can use the amount of Le 210 inventory in the sediment to give you a sense of how much exchange of water there must be shoreward in this system that can uh that is time integrated by the lead 210 signal in the bottom so now I’m switching to talking about the amount of lead 210 in this in the deposit and what you can see is this inventory here which is the total amount of lead 210 uh in the sediment in DPM per centimeter squared of bottom area is shown here you can see these Contours showing you massive amounts of lead 210 uh that is clearly has its origin offshore and the lead 210 is moving onshore up in upwell water and being extracted in that highly turbid water which you saw earlier by the particles that are resuspended uh from the bottom and they move uh shoreward so there’s a a lowering of lead 210 inventory now the amount of lead 210 inventory that you’d expect based on the atmospheric inputs of lead 210 to this region are are just of the order of 10 DPM per centimeter squared or so and so there’s a couple of things I want you to notice here one is that the surface activity of lead 210 despite the fact that there’s a huge of lead 210 here from offshore that’s being upwelled onto the onto the shelf and moving across the shelf that the activity of lead 210 in the surface shown here in DPM per gram is pretty homogeneous across the Shelf in other words the surface material here is moving back and forth with tidal excursions of the bottom water and being homogenized but the amount that’s of lead two10 that’s actually building up in the sediment is quite different from place to place as the inventory uh varies substantially so the lead 210 is up weld from bottom water here moves onto the Shelf is scavenged accumulates in the bottom and we have a record then of the total amount of lead to10 that’s upwell and what demaster and co-workers did was relate that up weld water to the onshore nutrient flux and we able to show that one the volume of water up well is about 5 to 10 times the Amazon River flow and that it supplies 40 to 80% of the nitrogen that’s needed um on the shelf by phytoplank in activity about 50% of the phosphorus and about 20% of the Silicon so this is a record stored in the B bottom of the integrated onshore movement of water uh that’s now processed that now stimulates primary production the products of which also move shoreward as they fall into the bottom water and are uh entrained into those mobile uh bed regions that I mentioned earlier so about five to 10 times the Amazon River flow is moved on Shore as shown by this Tracer now I want to talk a more about what happens to the material Downstream as it moves from the Amazon River mouth along the coastline which it does and in a series of what are called Mud Banks essentially uh deposits of mud that look like this in a particular uh site you can see a mudbank complex here they’re about 60 to 80 kilometers long and about 30 kilometers across moving into the ocean and they’re flooded with water uh tily twice a day with a couple of meters of water depth here you can see the mud deposits are exposed subari here and uh so this is one such mudbank complex and this is a highly Dynamic Coastline this is actually a a a a figure showing the position of the coastline as a function of decade here 1950 1975 and 2010 and the the oscillations of the coastline and the migrations of mud Banks along it are associ iated with multi- decadal forcing so a lot of the coastal interactions I’m talking about today have longer term uh frequencies associated with the patterns of exchange that occur along them and so you should be aware that there are decal in this particular case decal variations in in a lot of these processes I’m referring to and this is one of those exposed mudbanks here so this is a blowup of part of a mudbank uh along the French gyana coast and the scale here for it now going I just want to say a few more words about mud Banks before I get to another system here uh which will be the Gulf of Papa because these represent what’s important to realize is that the entrainment of water offshore does not just stop at the Delta uh proximal Delta regions of the Amazon but actually end all along the coastline as these mud waves move and migrate along the coastline and so we have mobile mud here we have mud that’s stabilized for different periods of time as you’ll see in a moment here as shown by lead 210 so here’s a mud Bank just migrating from uh the Amazon Downstream uh toward French along French Guyana toward the oronoko river now this is uh these these are characteristic types of profiles of thorium 234 and Lead 210 that you find in these mud Banks and one of the things I want you to realize right away is it is absolutely remarkable that you find any excess thorium 234 in the this mud thorium 234 has a 24-day halflife it’s produced by uranium in the water column ocean associated with salinity in the ocean and so um you need water you need sea water to produce uh thorm 234 that can be extracted into the seabed and the fact that we we see um I’m trying to drag all these pictures I have of myself and other people someplace else sorry there we go that’s better uh now you can see the this is the inventory of thori 234 in one of these mud Banks it’s showing you get 3 to 9 DPM per cenm squared and let me uh tell you that about if you have about 10 meters of water depth at every centimeter squared for a typical salinity open ocean solinity of uh water depth you’d expect about 2.8 DPM per uh centimeter squared uh inventory so now we just have a couple of meters just for part of the day of water here that could produce thorium 234 and in fact we get 3 to n DPM per centimet squared so the bottom line is that there’s a lot more thorium stored in these mud banks that could ever be produced by the water anywhere close to those mud Banks and this is showing you that particles are being advected shoreward toward the mud Banks they’re not just moving along the shoreline with the mud Banks but they’re being entrained in the mud Banks from offshore Shore and here’s lead 210 uh showing you homogenized lead 210 so this shows you what I’m talking about here’s the water colum uh thorium 234 being produced in the water column in proportion to the amount of uranium 238 the water depth determines the underlying inventory uh coupled with the solinity of the overlying water so what’s happening in these systems is that the thorium 234 produced in deeper water offshore is being aded with particles to the inshore regions and accumulating an inventory inshore that’s far greater than you would expect based just on the overlying water so this is one of these tracers we have of uh of reactive particles now I’m going to show you something from long Allen sound uh which is different obviously than uh the Amazon but this shows you that the amount of remineralized ammonium in sediments of Long Island Sound tracks uh very closely the excess thorium inventory in the uh bottom sediments that you see there and this is typical of essentially everywhere the thorium 234 Associates with reactive particles and so where we see thorium 234 excess uh in the bottom sediments we inevitably see uh excess reactive carbon coming in so back to the mud BL Banks back to uh lead 210 the outer part of a mud bank has is high in lead 210 the inner part of mud Banks is uh low because it’s stabilized for halflife or so of lead 210 so the activity of lead 210 in the inner part of a mud bank has decayed Away by about a time scale about 30 years or so as it’s stabilized it will be remobilized and reemerge into the outer mud bank and migrate down the down the system uh as mud moves from the uh the Amazon to the Oronoco now another indicator of this entrainment of reactive particles from offshore is the remineralization of of uh organic matter in the sediments of of mudbanks and this shows you the stochiometric relationship of dissolved ammonium in the poor water with dissolve uh DIC uh in the poor water and the ratio that we typically see in these mud Banks is very close to the Redfield ratio showing you that Marine organic matter uh is being entrained into this mud all along the way as it moves uh from the head of the Amazon River to uh the Oronoco now this is a schematic diagram of this system that I like to show showing you the Amazon River here the oronoko river here mud is accumulating proc approximately here off the Amazon River uh creating those thick mix layers you saw earlier and part of it’s migrating in these mud Banks along Shore and I want to emphasize that not only do we get as shown by the seawifs images and there chlorophyll a color bands cord exchange of material but it’s the lead 210 and thorium 234 shows you get shw exchange of the highly productive uh uh excuse me highly reactive particles produced in the highly productive regions offshore coming back on to shore now we can quantify some of these entrainment here and and this is where the cosmogenic radc uh silicon 32 could be used can be used and uh so again we have material which we uh uh in a model we can say is generated here and it if it moves along the coastline uh just from a source region here without any entrainment Downstream we can expect one pattern of radio uh chemical uh activity changes as it moves from here to there and Decay takes place and if it’s receiving material all along the way we can expect another pattern so I’m not going to go into these equations but they represent a advected uh type model of material aterial from a source region here along the coastline uh progressively here with decay of a radio nuclei that’s sourced here in the Delta and uh and you can measure it along this pattern if it if it’s not replenished by entrainment of material from offshore the radio nucleid will Decay and become lower and so here are uh uh silicon 32 activity patterns uh along the Amazon dispersal system with the Amazon Delta proximal Delta here the uh French Guyana Coastline about here about 700 600 700 kilometers Downstream and shy Ramen uh measured the lead 210 excuse me the Silicon 32 activity here and here and uh what you can see is if there were no introduction of new biogenic silica in caring silicon 32 into the system we would have seen a simple Decay away of the Silicon 32 as it moved along the coast laterally toward the Oronoco but in fact you see extremely high silicon 32 along the uh amaz along the French cyana Coast telling you that there’s entrainment all along the way of new biogenic reactive opal and silica to sustain this kind of activity so this is this is one example then of this entrainment that occurs using a totally different Tracer than the lead 210 and I wanted to emphasize because this group is particularly uh interested in carbon that all of that entrainment involves carbon moving back to the Shoreline where it’s then remineralized now in the last part of this talk uh I’m going to talk about Popa Guinea which is an incredible Place uh in terms of river inputs to the ocean and Delta formation there’s a whole range of different rivers that surround Papa New Guinea so this is a high relief uh system uh High delivery uh mountainous region that uh supplies tremendous amounts of material to the Sea and if you sum all these arrows which is another one of these unidirectional mman type uh diagram showing the amount of metric tons millions of metric tons of sediment brought to the ocean in the different regions of Papa New Guinea if you go all around the island here or the mini continent I suppose of Papa New Guinea and New Guinea uh it’s about equal to about 1.5 Amazon so the integrated amount of material coming off this area is greater than the Amazon River now I’m going to talk about this area right here and illustrate some of the processes I’ve tried to talk about earlier with the Amazon and show that they occur here as well in the Gulf of Papa so here’s a blow up of the Gulf of Papa there are multiple Rivers entering uh the Gulf of Papa the fly River and the parari rivers are the two largest the fly is on the uh uh uh here you can see that the fly River mouth this kind of funnel shape shows you the tide uh tides are high here and the lack of funnel shape morphology off the parari show you that the tides are lower on this sides of the gulf now one of the things that characterizes the gulf is that all these river systems uh Supply material which blends together to form a prograding clinoform Delta out through the gulf so here are uh bimetric patterns from the inshore offshore along this G transect here and another one along the H transect here and I’m going to show you data from different stations up on the uh inner shelf here and less than 20 uh meters of water and five 8 15 14 meters of water and a couple from 50 and 48 meters along this Delta just to convince you this is a programing Clin form here is a seismic uh profile across uh the uh the Gul of Papa shelf in the middle here essentially along between the G and H transects that I just showed you here’s about 20 meters of water here going into deep water and so you can see this classic uh type of of kinor structure with four set bottom set beds and top set beds here and if you look functionally at how this system works uh we have uh River inputs here on the left Mangrove coastlines here Sandy sediments which I’m not talking about today uh which accumulate in Shore and then movement of material out onto the top set and then a further movement of material where it largely accumulates in that four set beds of that kinfor structure but what I want to talk and so again we’re going to be thinking about this is material moving from land to the kinor deposits and then to the eventual um uh uh bottom set and fouret deposits here offshore and we have these these vectors showing the direction of net transport of sediment but I want to concentrate on these Aras going back onto this system that bring back into this top set region where we have mobile beds some of the material that’s formed offshore so here’s a cross-section an idealized CR cross-section of this kind of system with mobile top set beds here shown schematically and shown schematically here with that kind of lead to10 mix layer fluid mud uh overlying uh unconformably slowly accumulating sediments here we have biot turbet sediments in the forset region and Mangrove uh deposits here these are xradia graphs across this kind of system showing you the sedimentary structures that you see associated with these different depositional regimes you can see the This is 40 cmers depth in the sediment 80 at a site uh in the pi Inlet and 10 meters of water here we have uh eight meters of water along that GH transc from the sediment water interface to 40 uh CM uh depth in the SIM and both these sites you can see are highly laminated showing you physical dominance of the processes that control sediment deposition as you get into deeper water you pick up biogenic structures in the sediment here again this is 30 cmers 60 uh cmers here and 60 and 90 cmers here in a this should say 50 gh50 meters of water depth and so you can see that translation from physically controlled depositional regimes to biologically mixed and reworked uh deposits here offshore now one of the things that characterizes this system is that there’s a a huge seasonal variability in the wind with monsoon winds uh going offshore and trade winds going onshore seasonally and these Southeast trades kick up the sediment mix the sediment move the sediment shoreward uh in this system extremely high winds it’s very difficult to work in the Gulf during the southeast trades because sustained wind speeds are are 20 25 knots are typical so if you look at the color of the sediment in a 14 meter uh site in the middle of the gulf here on the top set this shows you the consequences of the wind driven uh wave and tidal activity in the re mobilization of the sediment so this is a a core that’s exactly the same actually as the xradio graph you were just looking at from uh wasn’t exactly the same same uh scaling as the x-r graphs you were just seeing here’s an unconformity here’s a mixed layer in this uh deposit here and uh this shows you the lead 210 reworked layer at this particular SES about thir 30 to 40 cm thick and and uh and it’s taken this particular core was taken during the southeast trades now if you look at the thorium 234 in that core that you were just looking at you can see that there’s very high thorium 234 in it penetrating to about uh 8 to 10 cmers depth that’s during the southeast trades and the inventory during that time of the thorium is 4.8 DPM per cenim squared here’s inventory and the I profile during the monsoon when the sediment has been sitting there uh unre worked or not are relatively stable it accumulates a much lower amount of thorium inventory about one DPM per centimeter squared and its penetration is just uh a couple of centimeters so what happens when the sediment sits there after it’s been reworked during as for example in this case by the trade winds is the sediment goes through a transition of diagenetic Redux reactions and becomes stuck for long periods of time in Iron reducing uh stages of diagenetic Redux succession and so the poor water patterns of dissolved iron that correspond to the two seasons I was just showing you at this site look like like this during the southeast trades when the sediment has been kicked up and oxidized uh there’s very little Ferris iron in the solid relative to the total uh reactive iron in the solid maybe 10 to 20% of the irons reduced in that uh light colored layer you were just looking at at the same time there’s very little iron in the poe water this is the dissolved iron in the poe water now if the sediment sits there for a while the iron gets reduced and so after a number of months there a lot more Ferris iron in that sediment and uh there’s much greater percentage of Ferris iron in the sediment before it’s reworked uh and presumably reoxidized during the next uh cycle of Trade Winds and during that time when it’s sitting there po water iron grows in to much higher levels and the change in the ferris iron composition from here to here would correspond to a CO2 or I see flux from the bottom of this system of about 30 to 45 moles per meter squar per day so in other words it’s really reactive sediment now if we look at these thorium 234 excess patterns in the Gulf uh as a function of the ratio of excess inventory to what you would expect based on water depth and a one-dimensional deposition of thorium 234 which is shown here by these Dash lines if you look at the amount of thorium versus water depth here across the shelf that I’ve just been showing you it’s clearly a lot of excess thorium 234 is accumulating inshore uh particularly during the trades and then during the monsoon where we only have one site here to compare with on this graph there’s much less accumulation so the point is during the trades there’s movement in particular of thorine 234 on Shore and it’s plastered on inshore uh during the um during the trades and entrained into the bottom sediments during that time and when that happens there’s oxygen exposure of the sediments reoxidized there’s metabolite exchange of the sediment as it’s as shown earlier by that radium data from the Amazon shelf and there’s entrainment of reactive carbon which enhances positive priming in the sediment now the lead 210 profiles uh excess lead 210 in this uh Gulf of Popa show the same thing here is the 0 to 10 meter bimetric depth 10 to 20 20 to 30 30 to 50 and this is the inventory of lead 210 here that we see in the sediment in the Gulf of Papa at multiple stations this is the number of stations that each one of these bars corresponds to in these different bimetric rang and this dash line here shows you the amount of lead 210 you’d expect if it just atmospheric input of lead 210 to this uh system so there’s a lot more lead 210 here as we also sh saw on the Amazon shelf than you would expect based on atmospheric input and I can’t go through and I won’t go through the calculation here but I followed a similar thing that Dave demaster has done on the Amazon shelf and the meong and there these lead to 10 uh inventories must be accounted for by about 10 to 12 times the river flux of water from offshore on into the Gulf of Papa so there’s a huge flux of water and particles as shown by the reactive uh thoram 234 from offshore to inshore in this system if we look in the sediment at remineralization of carbon in these sediments uh in the Gulf of Papa and we plot the sedimentary carbon isotope composition of the organic carbon uh in in the solid shown here on the Del C13 value of that carbon and the Del 14c value of that carbon we get patterns of the solid sediment like this if we look in the poe water that what’s actually being remineralized we get much heavier carbon in the DIC than in the solid carbon pool and much younger carbon and this reflects that entrainment of reactive carbon from this the ocean from the offshore regions inshore into the Gulf of Papua and it’s blended with the older carbon the more terrestrial carbon brought from the land and then diagenetically remineralize preferentially so that we get a diagenetic fractionation of the carbon in the sediment so I’m just going to summarize here I I know I’ve I’ve gone rapidly here and I’ve shown two different systems but I tried to emphasize today the fact that we have tracers available to us that can help us quantify phenomena like the onshore flux and entrainment of reactive carbon that we can use these traces to give us time scales and to demonstrate processes uh such as U as um digenetic fractionation of material brought from offshore inshore and the uh we can also use these tracers like radium for instance to show the offshore export of digenetic products like remineralized nutrients and so this is a series of sort of summary of what I’ve all the things I’ve mixed up here in my presentation but I hope you’ll take away that uh these are really complex four-dimensional systems they’re time dependent the three-dimensional in terms of material movement and onshore offshore movement of material it’s not just a progressive movement of material from the land to the Sea and the piling up of that material and big big deltaic deposits offshore there’s a lot of interactions with the ocean and we can quantify them with these Suites of tracers that I’ve tried to give you some examples of today so I’ll end it there uh I guess since I started 10 minutes late I’m not too badly overtime but I’m a little longer than I intended to be but I hope you take away the main points I was trying to get across here despite my fumbling around a bit so that’s it thanks Bob that was great um I now let’s open it up for questions so people can either um raise your hand if you have questions and we’ll unmute you um or we C you can put questions in the chat I also just um real quickly want to I forgot at the beginning to say while people are getting their questions together um I want to remind people that we will um The Leaky Deltas group will be having a workshop in March 2025 um in Baton Rouge um and so if you are interested in talking about carbon on Deltas I encourage you to sign up and Heather just put the link in the chat um to um express interest in it if you want to be kept in the loop um and with that um questions I should add that they’re they’re leaky both ways if you take a uh a bottom line here they’re leaky in all directions inore offshore along sure I have a question I can start us off if uh um no one’s raising their hands yet while people are thinking so I had a question about when you’re talking about integrated oxygen exposure times and I think I might have missed it in the last couple slides um H would do do you have a way to estimate oxygen exposure time or more like relative oxygen exposure time like for some particles get more compared to others or how are you estimating oxygen exposure time yeah that’s a that’s a really great question and and the fact of the matter is we can only estimate oxygen exposure time sort of in a minimal uh sense that is we can only uh classically it was it was uh originally uh Quantified using local diagenetic models and looking at the gradient of oxygen uh at one site into the sediment and determining how long material was in the oxic zone of sediments before it was buried at least for the first time buod and then subsequently people begin to try to incorporate the fact that particles don’t just Land one place and get buried there but they move continuously into the ocean so they’re resuspended and reexposed to oxygen many many times and in the case of um the uh let’s just say the Amazon the way you might ex uh estimate oxygen exposure time would be the time that particles have been in transit along the coast and so um that might be if you go from the Amazon to the oronoko it might it’s about a thousand years to do that so we uh that material is going over and over again it’s getting buried reduced and then it’s getting exed and moved into the mobile outer part of this stream of mud and move down Shore and so the minimum uh oxygen exposure time would be about a thousand years but actually it’s even longer than that it’s it’s going to be uh part of the time that it’s plastered onto the shoreline and then part of the time where it’s moved offshore and then back onshore so we can really only sort of quantify minimal estimates of oxygen exposure time and uh but what we know is um these the sedimentary Dynamics greatly enhances oxygen exposure time and that example I gave from the Gulf of Papa illustrates that where you have material sitting at a site reducing and then the trade wind forcing of the bottom currents and wave interaction with the bottom remobilized that sediment reexposed it to oxygen and essentially it’s been uh in a year it’s experienced another multiple uh uh periods of time of oxygen exposure so the point is it’s just not a one-way street it’s back and forth and back and forth just like my talk here so no yeah um Joe tamborski has a question and let’s see if he can uh and thanks for your answer Bob I appreciated it um see I think he can talk now hey Bob can you hear me I can great talk thanks for sharing I I’m curious if you were to go back to the the Millan diagram of all the vectors from the continents to the ocean you know you’ve kind of made the case that there’s the dissolves transport offshore but there’s this onshore transport of particles right so you know thinking about this globally for Global biogeochemical cycles how might you suggest we start to tackle these issues as a community and thinking more about parsing out this inshore versus offshore exchange of materials yes I think the U the first thing I think we need to think of is that these are interactive systems and we need uh to Quant ify those interactions using multiple tools on different time scales and um one of I I just gave one sort of subset of the tools emphasizing the a thorium uranium series tools that are available to us in addition to that cosmogenic uh new CL that shy specializes in and um this silicon 32 and all of these have different time scales all of them have different uh mean means of illustrating interactions so for example the radium is a great way of showing fluid and dissolve solute interactions with solids in different areas the thorium 234 is a great way of illustrating a reactive particle transport that’s associated with highly reactive newly formed organic matter in the system and where it ends up in in a given uh system and the ladu 10 and Ates over much longer time scales and uh but shows you sort of the mean behavior of a system on decal types of uh time scales and um so well I guess what I would say is we need to go into different systems with these sets of tools and not just say concentrate on uh measuring only carbon components of the system you need to use these tools in tandem with the carbon measurements in order to get the most out of how you can interpret carbon uh behavior in these Coastal systems so um I I’m a I guess I would say we need to take a multidisciplinary approach and by in this case in the disciplines of biogeochemistry not just the carbon group but the radiochemical group need to join together with the sedimentology groups uh to properly constrain each each of these systems which will differ great does that answer uh your question in a long-winded way yeah it does thank you and what a great motivation for leaky Deltas uh we have a question in the Q&A it’s from uh lingquan mu um and I’ll read it to you if that’s okay Bob yeah uh so great talk I’m new to radioisotopes but could you elaborate for example on how um from the 234 thorium or lead 210 profiles we can draw conclusions that the sediments came from offshore ined onshore instead of the other way around in the Gulf of Papa yes I’ll try to explain so uh here is uh a schematic diagram showing you exactly how I told you not to view a system which is but it’s one way we can view it to help us understand uh some of the things the controlling processes so if you’re in shallow water here here’s a coastline you’re in shallow water it might have low salinity it might have high salinity but uh uranium 238 which is the the uh parent for thorium 234 is present in the water column in proportion to salinity and also water depth so if we pick let’s say here if I make you dizzy with this laser pointer if you pick this uh water depth here there’s a certain depth of uranium uh overlying this Square centimeter or square meter of bottom there’s a depth uh it 10 every 10 meters of water depth uh is going to supply one liter of uranium 234 to every Square centimeter of bottom area so if if we uh have uranium 238 decay P to thorium 234 in the water column and the particles on which the thorium 234 attaches to it’s a highly reactive nuclei it attaches to the particles if they just went straight down from where they were attached to those particles we’d accumulate an inventory underneath this water depth that’s proportional to that water depth and the amount of uranium in it but what I was trying to show you is that when we look at the distribution of thorium which has to be formed here in in abundance because that’s where the uranium is if we look at the amount in the sediment we don’t see the uh small amount of thorium here that we might expect if the thorium only came from here we see a lot more thorium accumulating here than we would expect on that basis and it must come from here and it must move from where it’s formed where the uranium is where the salinity is where the water depth is offshore it must move onshore and accumulate there now that doesn’t mean you have net sedimentation although you can it just means you’ve exchanged the particles that were here with the particles that were here and the sediment water interface could stay at exactly the same position but the particles that were originally here have moved up and out and the particles that were originally here have moved in and down uh and so that’s where the image inventory tells you uh since it must form here that’s its source but it ends up here it has to be movement on Shore and entrainment into the bottom uh sediments to be buried there okay I hope that answers that qu and the same rule would apply to lead 210 as well but in a different uh way because lead 210 is sourced from the atmosphere from radon 220 two and from radium 226 in the water column offshore so I hope that gives you at least qualitative sense of how you can use inventory buildup patterns related to where something has to come from uh to show you that it’s not just one-dimensional that it’s a multi-dimensional exchange of particles from one place to another and I I didn’t spend much time uh in fact I spent only one slide showing you that thorium 234 attaches to because of its halflife it attaches to laile material typically plantonic material offshore and that tracks the reactive carbon pool as it moves inshore so where the thorium inventory is high the carbon reactive inventory is inevitably high as well this is the exact same thing that people use to estimate primary production in the open ocean using thorium 234 and sediment traps that they use the ratio of thorium 234 excess in Plankton accumulating in sediment traps offshore to tell you how much uh Plankton export there was from a surface water region into the sediment trap that you’re looking at and that’s a really a one-dimensional use of that Tracer and I’m using it in a multi-dimensional use using the inventory does does that answer your question um so they wrote In The Q yes it does thank you good uh yeah no that was a great explanation um I have a follow-up question sort of or shley do you want to go first I already asked a question I know if you have a followup please please go ahead well it’s actually not a followup it’s follow up to Joe’s question a bit oh me too so you go for it go for it I already asked I can follow so Bob am I can you all hear me okay um your little eoe talk slowly okay uh Bob what’s your opinion about the contribution of uh groundwater groundwater discharge to Regions like French gyana where you do have those exposures of you know in the erosional um edge of the mud Banks where you will have exposure of a lot of Sandy permeable um sediment that then has some connectivity offshore do you have a do you have you thought about that or do you opinion about yeah relative contribution might be no I don’t know and I don’t think anybody knows for that system okay and that you know that’s one of the holes in our our knowledge of these Coastal systems is the the groundwater inputs and that’s again where these radio nucle tracers in particular the radium Isotopes can provide ex you know really good insight into what these uh uh inputs must be and what so the the radium uh I was interpreting it the way uh Billy Moore did in that earlier work that a lot of that radium 224 in that in that system of the Amazon is has to come from remobilization of the bottom sediments and not likely from groundwater in that system although there’s certainly some groundwater input through the Sandy uh areas of the the Amazon mouth and in the Gulf of Papa there must be some from the the Sandy there’s extensive sand deposits in the inner Gulf of popwood that I didn’t talk about um and they almost certainly represent sources of of groundwater input now I don’t think that that would affect the lead to10 uh and that in the way I was looking at it uh in those areas although you know you could have a lot of radium 226 coming in there and you know and if it sat there in those areas and decayed in those areas long enough you could affect the lead 210 but generally speaking we uh ignore that contribution for that particular nuc clide but yeah that’s a great question that’s a wide open area it certainly has speedback on uh carbon cycling by nutrient inputs that would not otherwise be be there it’s another complexity that I think when you go into a system you need to have that in your toolbox of how you are going to address uh nutrient cycling and Elemental cycling generally in the in in a coastal area so I’m trying to I’m trying to open up the the way we go and attack studying a system you know you go in if you go in saying I’m can do an inshore offshore transect and and nail down everything you’re liable to be way off you know you you’ve got to think of things three and four dimensionally yeah it sounds like you’re that is you’re suggesting another very large uh project like amets to go to some of these systems which I appreciate thank you for for you and this group obviously I won’t be invol St you never know I I’ll go ahead and ask a question so I was curious so um building on Joe’s question these are two Deltas the Amazon is especially is a very large somewhat end member of a Delta and so I was curious if you’ve thought about to what extent this might be occurring on in other regions and like what um other end members it seems like if you were trying to make like a some sort of categorical diagram or thinking of end members you might have the Amazon on one end but then you also might have um that could categorize systems by like how much if you’re thinking about carbon budgets like how much productivity is happening offshore that has the potential to be brought in and then also something about like the physical oceanography that can actually bring this carbon in Shore are there I don’t know other things you might think about I mean about extrapolating the results here to other systems are thinking about how to categorize this yeah so it’s quite clear that not every Sy not every system behaves the same uh and certainly not the same as the Amazon or even the Gulf of Pop uh Clin ofform complex that I was talking about um so just to go back sorry about this um so if you look at these numbers here you can see the Gulf of Papa is about that’s you know that’s of order of the Mississippi or bigger in terms of the amount of material that’s brought in here so uh that’s an important single system but how do these other uh uh systems even around this single Island behave there’s no uh broad shelf here to bring in and uh bring in lots of water across the shelf on this Northshore uh area of Papa New Guinea so that around this single Island there’s a a huge range of different types of systems that one could go in and probably see very uh different kinds of results from what I was showing you now now the balance that I came up with for this using lead 210 is similar to what uh demaster and others got for the Amazon in terms of about 10 to 12 times the the river flow into this area but the Master’s also done the same kind of balance for the meong Delta and and there it’s only about two times the Mong water flow uh that that they were able to come up with so it’s quite clear that the entrainment of water U off different systems can vary substantially it’s not always going to be 10 times or five times uh it might be much smaller and um so I think 10 is is about what you’d expect in an estuary uh generally so these systems kind of behave like estuaries in many ways and so when it has a nicely expressed esterine um physical oceanography basically that then it the salt balance gives you this kind of factor of 10 you know shoreward Flow versus offshore and um so yeah no I agree a lot of your talk made me think of estuaries because even like within an estuary even if it’s an esterine source of carbon if it’s formed in a surface layer and sinking and then gets ined just further up the Estuary even if it’s not technically an offshore source so right um anyways thanks any other questions okay go for it well I have two I just wanted to query you Bob and the rest of the community or the attendees that are left uh the first is do you to follow up on Julia’s question is there an endmember locations that you think we should perhaps focus on to fill in a gap um between this uh what is it this low yield passive margin versus what you focused on this high yield well I guess the active margin would be on the north side of of ho new begin um is there another system I mean the one that comes to mind would be B of Bengal but is there another one that you think Could you um I for some reason I’m having a really tough time hearing you um I think I can paraphrase yeah the question was is there a spe more of a specific system you’d recommend as an end member um and sorry I was trying to like do some logistics for the seminar so I didn’t actually hear all the question but it was about um any specific recommendations for an end member study site is that fair shely that’s right I and think about as of act yeah but and then I think there’s also part of the question was then also maybe like just within Papa New Guinea you could use different sites as an act as different end members well Papa New Guinea is a great place to find multiple different kinds of uh of n members if you will because as you go around this island you find uh you know essentially active margin types that versus a passive margin type of broad shelf very narrow shelf here up U on the North Coast and uh here you can the rfr Shelf uh SE is a carbonate Rich sea so here you have Rivers empting into a carbonate platform kind of environment so that might be a great lower paleo uh type of model system um to go after um so I’m a big fan of this uh this place in the world as uh uh to to pick different parts of it to study and compare as multiple end members so that’s a good one there’s only a few uh places in the world that haven’t been drastically Modified by people uh in terms of U water balances and set balances and one that Steve Keel and others have been studying recently is the iaati system um it’s the last system in Asia that hasn’t been extensively damned uh it will be damned in the next five years or so um even more than it has a few small dams on it now but I guess it will be damned in the way that the Chong Jong is damned whether the Three Gorges type Dam which completely Alters the these Coastal sedimentary processes from what they were um so yeah there there’s there’s I would go around and and uh this is a great place because there’s not it’s not damned so that’s one thing that I think of if you want a system that gives you some insight into how things behaved in geologic time you need to go to places that haven’t been totally Modified by people um and that’s hard to find these days as I just alluded to and in Asia you know the last one standing is the AR arati B system and U so I don’t know I want to acknowledge that we’re over time and so we understand if you have to go um thanks Heather for reminding us um is it okay if I still ask your question it’s the acknowledgement that some people might have to go okay or do you want it answer it offline you’re welcome want um so in the chat Heather put a question um so you you bring a tremendous toolbox of tracers and other observing approaches for studying these systems working with hydrodynamic and sediment transport models seems like a promising way to get at physical entrainment and transport of particles on and offshore what is the best way to collaborate with modelers especially as we consider the interactions we want to Foster at the upcoming leaky Deltas Workshop yeah so um I so I’ll go back to the the ARA system which I just mentioned and um that’s an example where there’s been a really good interaction between people like Courtney Harris who uh specializes in physical uh oceanographic modeling and sediment transport modeling uh coupled to um Marine geology and sedimentology observations uh by Steve Keel and others in the same system and so I think having people like uh her who are really amable to um interacting with crazy biogeochemist types uh that that’s our sedimentologists that that’s what you need you need people that like working with each other and are interested in what each other uh brings to a problem and uh and in solving and uh uh that is realizing greater understanding and quantification and predictability uh uh in these systems so um I would say invite people like like Courtney to participate in these workshops and uh and uh some of it’s pretty amazing yeah what they come up with numerical modeling is really it’s great at producing things that look incredibly realistic even if they’re not realistic but uh but it’s a wonderful way of quantifying these things and use together in tandem with the observations really powerful I is that is that okay Heather thanks Bob really appreciate that um looks like you got one more question here if you wanna allow Jen to unmute herself and ask that um yeah go ahead Jen I am unmuted I had a question in the Q&A because my chat’s disabled but I think Heather’s question covered it so okay thank you I think there’s one more yeah I just reopened the Q&A again so there’s one from Leu giosan um how about Systems off the Andes closer and less dangerous nowadays most not damned yet well what was could you repeat that please yeah so the question was about um I’m going to put some words in their mouth that maybe like the systems off of the Andes would also be good systems to consider working on um they might be closer and less dangerous um now in in present uh day um and most aren’t damned yet off the Andes so on the the west coast of South America is that what we’re I believe so yeah um of course there’s been a lot of work in the upwelling boundary of that area um I’m not aware of any major river work um there’s been in terms of active small mountainous River work there’s been a lot of oh yeah hiob this nice to see you hey you I can’t see you I can only see your name printed out I’m not sure if I’m allowed it’s or I don’t know how to use it there were some security issues with some previous thing um seminars not ours my name was uh my name is Le you Joan Just for future reference thank you nice to see you Bob uh I’m thinking about the rivers coming from Colombia lots of lots of rain coming down and building Deltas that have been studied recently by uh by Juan Restrepo for example lots of lots of Deltas that are undiscovered and they are not damned and irawadi I’ve been working on iradi just before the war started and papa new guine is is very far and very dangerous yeah yeah that actually in my own case I I had tried to work in the arati uh but then the coup happened and that was that was that and so that is an issue there you’re exactly right I’m really not familiar with the the rivers that drain Colombia so I can’t I think those are the kinds of things and points that you make that should be brought out put on the table and discussed by people yes and that’s that’s really great to know I didn’t I wasn’t um aware of that opportunity that’s the role of the workshop that’s right now I would challenge you about the dangers of Pop of popping New Guinea because the um once you get out of Port Moresby or Lei out of the Cities it’s actually U quite quite safe to work in that’s true but you wouldn’t challenge me of how far they are that’s true that’s true thank you to the workshop organizers there this really good series thank you and thank you for tending and bringing questions thanks Lev you and I wish I could see see you I can only see your written name but I I have more white here so um any last questions okay well let’s give a huge thank you again to Bob um I know it’s not very satisfying when only one person is clapping um but thank you a huge thank you for that great talk and um for the Q&A thank you for to everyone for participating in the Q&A um with that we’ll see you at next seminar which I think is in the fall um or at the annual OCB meeting and in the meantime please sign up for the uh Workshop the expressing interest in the workshop that will be in March yes please do fill out that form um thank you for having me thank you for listening appreciate it that was great thank you thanks everyone byebye