What DOES control the neodymium signature we see in sediments?
Check out the full paper here
First things first,
Why do we care about neodymium?
-We use neodymium isotopes to reconstruct what ocean circulation has done in the past which gives us some insight into how ocean circulation may be impacted by modern climate change (see how we used neodymium isotopes for the world 55 million years ago here).
Ok, so if we’re already using it, what’s the problem?
– Various observations both in the ocean and from computer models suggest we don’t understand the elemental budget of neodymium in the ocean. Just like a checking account, the budget needs to balance: it becomes a problem when in=out! But isotope data suggests that we don’t know where most of the ‘in’ is coming from!
My blogging on the first parts of this project was essentially non existent, so what you need to know from parts 1 & 2 for the update on part 3:
Part 1- Does the sediment (mud!) supply neodymium to the ocean?
The short answer: yes, and likely quite a bit! We estimated how much neodymium is released into the water column (shown here by black arrows) from the mud based on the concentration of neodymium in the pore fluid (shown here by the black lines in the shaded box)
Why does this matter?
Traditionally, the sediment has been considered only an ‘out’ term for neodymium in the ocean. But, if the sediment is actually an ‘in’ term, we have a way to balance the budget!
You can also access the highlights from this part on the GEOTRACES website here
Part 2– What does this new ‘in’ term mean for the isotopes of neodymium in the oceans? (the ones we use to look at circulation in the past!)
First of all, it means that the neodymium isotopes in a water mass aren’t conservative- that is, they can change as the water mass moves IF the water mass is deep enough to be in contact with the sediments. If the “signature” of the water mass can change, it cannot be strictly interpreted as an indication of where the water mass originated. This presents challenges to using neodymium isotopes as a tracer…
Ok so now Part 3: How does the sediment source work?
This is essential in applying what we learned in part 2 (neodymium isotopes can change as a water mass moves due to an additional ‘in’ term of neodymium from the sediments) to models looking at ocean circulation because it provides a starting point to identify where these modifications to the water mass can happen (that is what conditions are most likely to change the neodymium isotopes!).
For reconstructing ocean circulation we look at an “authigenic” part of the sediments (the one we think most represents seawater! shown here by striped lines in our sediment particle schematic). We get at this part by mildly leaching sediments (exposing them to a weak solution of chemicals) and measuring what comes off. So for part 3, we also look at how the record from authigenic part of the sediments actually differs from seawater it is thought to record, the pore water (the water in between sediment particles), and the rest of the sediment (brown portion in schematic).
What did we find?
The relationship between the isotopes in the authigenic part, seawater, pore water, and the rest of the sediment varies by location. The largest variation is near shore corresponding to our smallest predicted flux of neodymium from the mud into the overlying water.
What does this mean?
Based on these locations, we think the authigenic part is constantly interacting with pore water-therefore driving the source of neodymium to the water from the sediments that we saw in part 1. Because the interaction is continuous, the sediments further from shore that have been reacting longer will have more authigenic parts and these authigenic parts will look more like pore water. Near-shore has added complexity. Near-shore we get very ‘young’ sediments, freshly washed in from the nearby Oregon coast with a smaller authigenic fraction that have not had long to react with the pore water. These sediments can contain some material that is very reactive (and is therefore gone by the time we get into the further off shore sites). This reactive material can have a very different neodymium isotope signature from the surrounding sediment, and because it is so reactive will preferentially influence the pore fluid signature. This may explain why a lot of the ocean observations that find issues with the neodymium budget are from near shore regions!
If you want to know more, the full paper is available FREE (until August) in Earth and Planetary Science Letters!