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1. Impact of the Elemental Composition of Exported Organic Matter on the Observed Dissolved Nutrient and Trace Element Distributions in the Upper Layer of the Ocean

   https://doi.org/2020GB006902  

   Quay, Paul (September 2021, Global biogeochemical cycles)

   Mikaloff Fletcher, Sara (Ed.)

   Systematic regional variations in the ratio of nutrient depth gradients of dissolved inorganic carbon (ΔDIC): nitrate (ΔNO3): phosphate (ΔPO4) in the upper layer (300m) of the Pacific Ocean are observed. Regional variations in the ΔDIC/ΔNO3/ΔPO4 are primarily the result of three processes, that is, the C/N/P of organic matter (OM) being exported and subsequently degraded, nitrogen fixation and air-sea CO2 gas exchange. The link between the observed dissolved ΔDIC/ΔNO3/ΔPO4 and the C/N/P of exported OM is established using surface layer dissolved DIC, NO3 and PO4 budgets. These budgets, in turn, provide a means to indirectly estimate the C/N/P of OM being exported from the surface layer of the ocean. The indirectly estimated C/N/P of exported OM reach maxima in the subtropical gyres at 177/22/1 that is significantly greater than the Redfield ratio and a minimum in the equatorial ocean at 109/16/1 with both results agreeing with available observed particle C/N/P and ocean biogeochemical models. The budget approach was applied to a bioactive trace element (TE) using the measured dissolved Cadmium (Cd) to PO4 gradients to estimate the Cd/P of exported OM in the Pacific Ocean. Combining the budget method with the availability of high-quality dissolved nutrient and trace element data collected during the GOSHIP and GEOTRACES programs could potentially provide estimates of the C/N/P/TE of exported OM on global ocean scales which would significantly improve our understanding of the link between the ocean’s biological pump and dissolved nutrient distributions in the upper ocean. 

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2. Regional Patterns of Organic Matter Export Rates Along the GEOTRACES Pacific Meridional Transect GP15

   https://doi.org/10.1029/2024GB008277  

   Quay, Paul; Stephens, Mark (January 2025, Global Biogeochemical Cycles)

   Abstract Export rates of organic matter (OM) were determined based on PO₄³⁻, NO₃⁻and O₂budgets during GEOTRACES cruise GP15 in the Pacific Ocean that crossed subpolar, subtropical and equatorial regimes. Lowest OM export rates at 3–5 mmol C/m²/yr were found in the subtropical regions and highest rates at 9–12 mmol C/m²/yr were found in the equatorial and subpolar regions. Satellite based OM export rates showed similar regional trends but with a significantly larger range. The budget and satellite‐based OM export rates were 3–15× higher than estimates of particle loss rates based on²³⁴Th and sediment trap collections, with the differences primarily due to non‐particle forms of OM export and different integration times of methods. The efficiency of export varied from 0.1 to 0.3, with the lowest efficiencies in the subtropics and highest efficiencies in the subpolar and equatorial regions. 

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3. Biological, Physical, and Atmospheric Controls on the Distribution of Cadmium and Its Isotopes in the Pacific Ocean

   https://doi.org/10.1029/2022GB007441  

   Sieber, Matthias; Lanning, Nathan T.; Bunnell, Zachary B.; Bian, Xiaopeng; Yang, Shun‐Chung; Marsay, Chris M.; Landing, William M.; Buck, Clifton S.; Fitzsimmons, Jessica N.; John, Seth G.; et al (January 2023, Global Biogeochemical Cycles)

   Abstract Despite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of data. Here, we present dissolved Cd and δ¹¹⁴Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ¹¹⁴Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ¹¹⁴Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO₄maximum and is thus a consequence of an integrated regeneration signal of Cd and PO₄at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol⁻¹), rather than being related to in situ removal processes in low‐oxygen waters. 

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4. The Importance of Reversible Scavenging for the Marine Zn Cycle Evidenced by the Distribution of Zinc and Its Isotopes in the Pacific Ocean

   https://doi.org/10.1029/2022JC019419  

   Sieber, M.; Lanning, N. T.; Bian, X.; Yang, S. ‐C.; Takano, S.; Sohrin, Y.; Weber, T. S.; Fitzsimmons, J. N.; John, S. G.; Conway, T. M. (April 2023, Journal of Geophysical Research: Oceans)

   Abstract The North Pacific has played an important role in ongoing discussions on the origin of the global correlation between oceanic dissolved Zn and Si, while data in the North Pacific have remained sparse. Here, we present dissolved Zn and δ⁶⁶Zn data from the US GEOTRACES GP15 meridional transect along 152°W from Alaska to the South Pacific. In the south (<20°N) Zn and Si exhibit a tight linear correlation reflecting strong Southern Ocean influence, while in the north (>20°N) an excess of Zn relative to Si in upper and intermediate waters is due to regeneration of Zn together with PO₄. Using a mechanistic model, we show that reversible scavenging is required as an additional process transferring Zn from the upper to the deep ocean, explaining the deep Zn maximum below the PO₄maximum. This mechanism applied for reversible scavenging also provides an explanation for the observed isotope distribution: (a) fractionation during ligand binding and subsequent removal of residual heavy Zn in the upper ocean, drives the upper ocean toward lower δ⁶⁶Zn, while (b) release of heavy Zn then coincides with the PO₄maximum where carrier particles regenerate, causing a mid‐depth δ⁶⁶Zn maximum. In the upper ocean, seasonal physical stratification is an additional important process influencing shallow δ⁶⁶Zn signals. At the global scale, this mechanism invoking fractionation during ligand binding coupled with reversible scavenging offers a global explanation for isotopically light Zn at shallow depths and corresponding elevated mid‐depth δ⁶⁶Zn signals, seen dominantly in ocean regions away from strong Southern Ocean control. 

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5. Global Contrasts Between Oceanic Cycling of Cadmium and Phosphate

   https://doi.org/10.1029/2021GB006952  

   Roshan, Saeed; DeVries, Tim (June 2021, Global Biogeochemical Cycles)

   Abstract Cadmium (Cd) is a trace metal whose distribution in the ocean bears a remarkable resemblance to the nutrient phosphate (PO₄³⁻). This resemblance has led to the use of Cd as a proxy for ocean nutrient cycling in paleoceanographic applications, but the processes governing the cycling of Cd in the modern ocean remain unclear. In this study, we use previously published Cd observations and an Artificial Neural Network to produce a dissolved Cd climatology that reproduces the observed subtle deviations between the Cd anddistributions. We use the Cd andclimatologies, along with an ocean circulation inverse model, to diagnose the biogeochemical sources and sinks of dissolved Cd and. Our calculations reveal that dissolved Cd, like, is removed in the surface ocean and has a source in the subsurface, consistent with the simultaneous incorporation of Cd andinto sinking organic particles. However, there are also contrasts between the cycling of dissolved Cd andIn particular, thesurface export ratio varies 8‐fold across latitudes, reaching highest values in the iron‐limited sub‐Antarctic Southern Ocean. This depletes Cd relative toin the low‐latitude thermocline while adding excess Cd to deep waters by the regeneration of Cd‐enriched particles. Also, Cd tends to regenerate slightly deeper thanin the subsurface ocean, and theregeneration ratio reaches a maximum at 700–1,500 m. These contrasts are responsible for a slight concavity in therelationship and should be considered when interpreting paleoceanographic Cd records. 

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