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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. A Tropical Cocktail of Organic Matter Sources: Variability in Supraglacial and Glacier Outflow Dissolved Organic Matter Composition and Age Across the Ecuadorian Andes

   https://doi.org/10.1029/2022JG007188  

   Holt, Amy D; Kellerman, Anne M; Battin, Tom I; McKenna, Amy M; Hood, Eran; Andino, Patricio; Crespo‐Pérez, Verónica; Peter, Hannes; Schön, Martina; De_Staercke, Vincent; et al (May 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract The biogeochemistry of rapidly retreating Andean glaciers is poorly understood, and Ecuadorian glacier dissolved organic matter (DOM) composition is unknown. This study examined molecular composition and carbon isotopes of DOM from supraglacial and outflow streams (n = 5 and 14, respectively) across five ice capped volcanoes in Ecuador. Compositional metrics were paired with streamwater isotope analyses (δ¹⁸O) to assess if outflow DOM composition was associated with regional precipitation gradients and thus an atmospheric origin of glacier DOM. Ecuadorian glacier outflows exported ancient, biolabile dissolved organic carbon (DOC), and DOM contained a high relative abundance (RA) of aliphatic and peptide‐like compounds (≥27%RA). Outflows were consistently more depleted in Δ¹⁴C‐DOC (i.e., older) compared to supraglacial streams (mean −195.2 and −61.3‰ respectively), perhaps due to integration of spatially heterogenous and variably aged DOM pools across the supraglacial environment, or incorporation of aged subglacial OM as runoff was routed to the outflow. Across Ecuador, Δ¹⁴C‐DOC enrichment was associated with decreased aromaticity of DOM, due to increased contributions of organic matter (OM) from microbial processes or atmospheric deposition of recently fixed and subsequently degraded OM (e.g., biomass burning byproducts). There was a regional gradient between glacier outflow DOM composition and streamwater δ¹⁸O, suggesting covariation between regional precipitation gradients and the DOM exported from glacier outflows. Ultimately, this highlights that atmospheric deposition may exert a control on glacier outflow DOM composition, suggesting regional air circulation patterns and precipitation sources in part determine the origins and quality of OM exported from glacier environments. 

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5. 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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