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1. The Sediment Green‐Blue Color Ratio as a Proxy for Biogenic Silica Productivity Along the Chilean Margin

   https://doi.org/10.1029/2022GC010350  

   Li, Chen; Clementi, Vincent J.; Bova, Samantha C.; Rosenthal, Yair; Childress, Laurel B.; Wright, James D.; Jian, Zhimin; Expedition 379T Scientists (September 2022, Geochemistry, Geophysics, Geosystems)

   Abstract Sediment cores recently collected from the Chilean Margin during D/VJOIDES ResolutionExpedition 379T (JR100) document variability in shipboard‐generated records of the green/blue (G/B) ratio. These changes show a strong coherence with benthic foraminiferal δ¹⁸O, Antarctic ice core records, and sediment lithology (e.g., higher diatom abundances in greener sediment intervals), suggesting a climate‐related control on the G/B. Here, we test the utility of G/B as a proxy for diatom productivity at Sites J1002 and J1007 by calibrating G/B to measured biogenic opal. Strong exponential correlations between measured opal% and the G/B were found at both sites. We use the empirical regressions to generate high‐resolution records of opal contents (opal%) on the Chilean Margin. Higher productivity tends to result in more reducing sedimentary conditions. Redox‐sensitive sedimentary U/Th generally co‐varies with the reconstructed opal% at both sites, supporting the association between sediment color, sedimentary U/Th, and productivity. Lastly, we calculated opal mass accumulation rate (MAR) at Site J1007 over the last ∼150,000 years. The G/B‐derived opal MAR record from Site J1007 largely tracks existing records derived from traditional wet‐alkaline digestion from the south and eastern equatorial Pacific (EEP) Ocean, with a common opal flux peak at ∼50 ka suggesting that increased diatom productivity in the EEP was likely driven by enhanced nutrient supply from the Southern Ocean rather than dust inputs as previously suggested. Collectively, our results identify the G/B ratio as a useful tool with the potential to generate reliable, high‐resolution paleoceanographic records that circumvent the traditionally laborious methodology. 

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2. Investigating the Nutrient Landscape in a Coastal Upwelling Region and Its Relationship to the Biological Carbon Pump

   https://doi.org/10.1029/2020GL087351  

   Stukel, M. R.; Barbeau, K. A. (March 2020, Geophysical Research Letters)

   Abstract We investigated nutrient patterns and their relationship to vertical carbon export using results from 38 Lagrangian experiments in the California Current Ecosystem. The dominant mode of variability reflected onshore‐offshore nutrient gradients. A secondary mode of variability was correlated with silica excess and dissolved iron and likely reflects regional patterns of iron limitation. The biological carbon pump was enhanced in high‐nutrient and Fe‐stressed regions. Patterns in the nutrient landscape proved to be better predictors of the vertical flux of sinking particles than contemporaneous measurements of net primary production. Our results suggest an important role for Fe‐stressed diatoms in vertical carbon flux. They also suggest that either preferential recycling of N or non‐Redfieldian nutrient uptake by diatoms may lead to high PO₄³⁻:NO₃⁻and Si(OH)₄:NO₃⁻ratios, following export of P‐ and Si‐enriched organic matter. Increased export following Fe stress may partially explain inverse relationships between net primary productivity and export efficiency. 

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3. A New Perspective on Past Export Production in the Subantarctic South Pacific for the Last ∼1.4 Myr

   https://doi.org/10.1029/2024PA004929  

   Toyos, María H; Lamy, Frank; Lange, Carina B; Abell, Jordan T; Lembke‐Jene, Lester; Arz, Helge W; Winckler, Gisela (February 2025, Paleoceanography and Paleoclimatology)

   Abstract Accurate reconstructions of export production in the Subantarctic Zone of the Southern Ocean are crucial for understanding the carbon cycle during Earth's past. However, due to the strong bottom water circulation of the Antarctic Circumpolar Current, sediment redistribution complicates age‐model‐derived bulk mass accumulation rates (BMAR). Here, we assess export production and its drivers over the past ∼1.4 Myr near the Drake Passage entrance using BMAR of biogenic barium, organic carbon, biogenic opal, calcium carbonate, and iron from sediment core PS97/093‐2, all of which are corrected for lateral sediment redistribution (corr‐BMAR). To quantify this correction, we explore the relationship between sortable silt as a bottom current strength proxy and²³⁰Th‐derived focusing factors as indicators of lateral redistribution of sediments, respectively. Our findings highlight peak Fe input prior and during glacials of the Mid‐Pleistocene Transition (MPT), likely driven by enhanced Patagonian weathering. The carbonate record indicates increased deep‐ocean corrosivity after around 1 Ma ago and displays a shift in the accumulation pattern post‐MPT, with only isolated peaks in some peak interglacials. The high carbonate values during MIS 11 likely relate toGephyrocapsacoccolithophore propagation, preceded and followed by prolonged carbonate dissolution periods, possibly linked to the Mid‐Brunhes Event. After the MPT, productivity proxies respond to glacial and interglacial intensity, with maxima found during MIS 16, MIS 11, MIS 5, and the Holocene, while minima occur during MIS 15–12. Our findings offer insights into long‐term productivity dynamics and their relationship to important climatic events over the past 1.4 Myr. 

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4. Hydrothermal Plumes Act as a Regional Boundary Sink of ²³⁰ Th in the Equatorial Pacific

   https://doi.org/10.1029/2024GB008219  

   Redmond, N A; Hayes, C T; Anderson, R F; Black, E E; Edwards, R L; Fleisher, M Q; Li, X; Lam, P J; Mateos, K C (April 2025, Global Biogeochemical Cycles)

   Abstract An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging,²³⁰Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved²³⁰Th violates the assumption that the flux of particulate²³⁰Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved²³⁰Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine²³⁰Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of²³⁰Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved²³⁰Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background²³⁰Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of²³⁰Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in²³⁰Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise. 

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5. Controls on Dissolved Barium and Radium‐226 Distributions in the Pacific Ocean Along GEOTRACES GP15

   https://doi.org/10.1029/2023GB008005  

   Le_Roy, Emilie; Charette, Matthew A; Henderson, Paul B; Shiller, Alan M; Moore, Willard S; Kemnitz, Nathaniel; Hammond, Douglas E; Horner, Tristan J (June 2024, Global Biogeochemical Cycles)

   Abstract Radium‐226(²²⁶Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full‐depth ocean section of²²⁶Ra activities (T_1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of²²⁶Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher²²⁶Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with²²⁶Ra inputs primarily resulting from the decay of²³⁰Th in sediments. Further, dissolved²²⁶Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non‐conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non‐conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of²²⁶Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non‐conservative processes that shape their respective distributions. 

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