Paper in press: Treatise on Geochemsitry
I’m pleased to share that our chapter, Tools to trace past productivity and ocean nutrients, has been published in the third edition of the Treatise on Geochemistry.
My involvement in this chapter began on a flight to the 2021 AGU Fall Meeting in New Orleans. I happened to be seated near Jesse Farmer, the lead author, who mentioned he’d been invited by volume editor Adina Paytan to contribute a chapter on productivity proxies—and asked if I’d like to collaborate. Coming off two comprehensive papers we’d co-authored earlier that year, Jesse was clearly ready for another ambitious project. And at over 22,000 words, this chapter certainly fits the bill.
What is this chapter about?
At its core, it’s a comprehensive review of geochemical tools used to reconstruct past ocean productivity and nutrient dynamics—key elements of the global carbon cycle (literally and figuratively). We set out to answer: What tools exist to track these processes, and what are their strengths and limitations? What we found was that, over the past 50 years, geochemists have been busy developing numerous proxies for tracing past productivity and nutrient conditions, resulting in a diverse and complex literature. To keep things manageable, we organized the review into three main sections.
1. Classification of geochemical tools
I took the lead on this part of the chapter. Our goal was to simplify the vast range of proxies into a clear, intuitive framework. Ultimately, we grouped proxies into two types:
- Output-based proxies, which directly or semi-directly reflect biological productivity, such as through the accumulation of organic carbon, opal, or barite.
- State-based proxies, which infer nutrient conditions or chemical states of past oceans, relying on indicators like nitrogen, silicon, or boron isotopes.
This classification updates classic work by Berger et al. (1994) and incorporates new proxies developed since then.
Output proxies are generally easier to measure and interpret—more sedimentation of a biogenic phase usually means higher productivity at that time. However, they reflect mostly local changes, are sensitive to preservation biases, and require precise constraints on sedimentation rates. State proxies offer broader spatial and temporal insights into ocean conditions—such as nutrient utilization or ocean pH—and don’t rely as heavily on sedimentation rates. However, they measure productivity indirectly, tend to be more challenging to analyze, and typically yield fewer data points, making interpretation less straightforward.
A key takeaway from our review is that combining output and state proxies often provides the most powerful insights, helping resolve ambiguities and strengthen conclusions. Personally, this section was a nice opportunity to revisit foundational studies in ocean chemistry. My aim was to make these explanations clear and accessible, starting from first principles to explain how each proxy works, why it’s effective, and its ideal interpretations—even before addressing complicating factors like diagenesis.
2. Proxy-by-proxy evaluation
The second section offers an in-depth exploration of individual proxies, detailing their geochemical principles, modern observations, and known limitations. Led primarily by Jennifer and Emma, this section is especially useful for those already familiar with the general literature but seeking deeper insight into specific proxies.
3. Case studies and multiproxy applications
Jesse led this final section, which provides concrete examples of how combining proxies has advanced our understanding of ocean biogeochemistry, particularly throughout the Cenozoic era. It highlights successful reconstructions and sets aspirational goals for the next decade, emphasizing the potential for improved methods and data.
Final thoughts
If there’s one message I hope readers take away from this chapter, it’s that all proxies are useful, but all have limitations. Every proxy has multiple possible interpretations and preservation biases. And yet, with the right context, a thoughtful combination of tools, and a hearty dose of humility, we can use them to uncover meaningful insights into Earth’s history. I hope this chapter serves as a valuable reference for anyone interested in marine productivity, nutrient cycling, and the ocean’s role in Earth’s carbon cycle. It was a fun and rewarding project—and it wouldn’t have come together without Jesse, who did a fantastic job keeping us organized, focused, and motivated throughout.
This chapter appears in Volume 5 (Earth’s Surface Envelope: Evolution Over Time) of the third edition of the Treatise on Geochemistry. There are some excellent chapters throughout—not just in this volume, but across all eight. The chapter is behind a paywall, but if you’d like to read it, feel free to email me—I’d be happy to send you a PDF.
Citation: Farmer, J. R., Fehrenbacher, J. S., Horner, T. J., & Kast, E. R. (2025). Tools to trace past productivity and ocean nutrients. In A. Paytan & A. Turchyn (Volume Eds.), A. Anbar & D. Weis (Eds.-in-Chief), Treatise on Geochemistry (3rd ed., Vol. 5, Earth’s Surface Envelope: Evolution Over Time, pp. 111–151). Elsevier, doi:10.1016/B978-0-323-99762-1.00039-5.