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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. Organic Matter Export Rates and the Pathways of Nutrient Supply in the Ocean

   https://doi.org/10.1029/2023GB007855  

   Quay, Paul ( July 2023 , Global Biogeochemical Cycles)

   Multiyear estimates of organic matter (OM) export based primarily on oxygen and dissolved inorganic carbon surface layer budgets applied basin‐wide for the Pacific, Atlantic, and S. Indian Oceans yield an inter‐basin range from 1 to 3 mol C/m²/yr with a global mean of 2.0 mol C/m²/yr (8.5 Gt C/yr). OM export rates per area in the Pacific and Atlantic oceans are twice than that in the Indian Ocean. The supply of nutrients from the Southern Ocean can potentially support ∼70% of the observed OM export in the ocean based on observed surface current velocities and PO₄distributions. Horizontal flux of PO₄and dissolved organic phosphorous in the surface layer can support ∼50%, 20%, and 15% of observed OM export in the Pacific, S. Indian and Atlantic oceans, respectively, with the remainder being supplied vertically from the subsurface. Potential utilization of unused surface PO₄in the subtropical gyre yields ∼0.1 mol C/m²/yr increase in OM export in the Pacific and Atlantic oceans but a ∼0.8 mol C/m²/yr increase in the S. Indian ocean suggesting that stronger nutrient limitation contributes to lower export rates observed in the Indian ocean.

    

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3. The influence of an in‐network lake on the timing, form, and magnitude of downstream dissolved organic carbon and nutrient flux

   https://doi.org/10.1002/2016WR019378  

   Kalinin, Alexey ; Covino, Tim ; McGlynn, Brian ( November 2016 , Water Resources Research)

   Within fluvial networks, lakes can be sinks or sources of dissolved organic carbon (DOC) and nutrients, yet the controls over sink‐source behavior remain unclear. We investigated the influence that an in‐network lake exerted on DOC and nutrient export. Our investigation consisted of: (1) injecting a conservative tracer to determine lake travel times and flow paths; (2) sampling lake inflow, outflow, and surrounding groundwater to determine water and nutrient budgets; and, (3) sampling internal lake profiles to ascertain in‐lake physico‐chemical patterns through time. Conservative tracer data indicated considerable in‐lake retention and combined with inflow‐outflow discharge measurements revealed a decoupling of kinematic and solute pulses. Nitrate (NO₃) was the dominant form of dissolved inorganic nitrogen (DIN) at lake inflow whereas ammonium (NH₄) became the dominant component at lake outflow. The lake was a sink for NO₃‐N and PO₄, but a source for NH₄‐N, DON, TDN, and DOC. We observed hydrologic controls on DOC concentrations and export patterns, but redox controls on DIN dynamics. Our results indicate that lakes within fluvial networks can be sources of dissolved organic material and reduced nitrogen (NH₄) while simultaneously being sinks for NO₃‐N and PO₄‐P. Determining controls on sink‐source behavior and the cumulative effect of lakes on DOC and nutrient budgets is a necessary first step toward improved understanding of the role of lakes in network‐ to regional‐scale dynamics.

    

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4. 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)

   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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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)

   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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