More Like this

1. Dissolved and Particulate Barium Distributions Along the US GEOTRACES North Atlantic and East Pacific Zonal Transects (GA03 and GP16): Global Implications for the Marine Barium Cycle

   https://doi.org/10.1029/2022GB007330  

   Rahman, Shaily ; Shiller, Alan M. ; Anderson, Robert F. ; Charette, Matthew A. ; Hayes, Christopher T. ; Gilbert, Melissa ; Grissom, Karen R. ; Lam, Phoebe J. ; Ohnemus, Daniel C. ; Pavia, Frank J. ; et al ( June 2022 , Global Biogeochemical Cycles)

   Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg⁻¹) and increase to 70–80 and 140–150 nmol kg⁻¹in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBa_xs) formation and dissolution rates were tracked by normalizing particulate excess²³⁰Th activities. Th‐normalized pBa_xsfluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m⁻² year⁻¹average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBa_xsburial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year⁻¹. Estuarine mixing processes may add another 3–13 Gmol year⁻¹. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year⁻¹), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance.

    

   more » « less
2. Biogeochemical Cycling of Dissolved Zinc in the Western Arctic (Arctic GEOTRACES GN01)

   https://doi.org/10.1029/2018GB005975  

   Jensen, L. T. ; Wyatt, N. J. ; Twining, B. S. ; Rauschenberg, S. ; Landing, W. M. ; Sherrell, R. M. ; Fitzsimmons, J. N. ( March 2019 , Global Biogeochemical Cycles)

   The biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient‐type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogeneous (sp.) with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by (1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes and (2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol,r² = 0.58) that is higher than the global slope (0.059 nmol/μmol,r² = 0.94) because of the shelf‐derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique preformed Zn:Si stoichiometries.

    

   more » « less
3. Origin and Accumulation of an Anthropogenic CO 2 and 13 C Suess Effect in the Arctic Ocean

   https://doi.org/10.1029/2019GB006423  

   Ko, Young Ho ; Quay, Paul D. ( February 2020 , Global Biogeochemical Cycles)

   We determined the impact of anthropogenic CO₂(C_ant) accumulation on the δ¹³C of dissolved inorganic carbon in the Arctic Ocean (i.e., the¹³C Suess effect) based on δ¹³C measurements during a GEOTRACES cruise in 2015. The δ¹³C decrease was estimated from the amount of C_antchange derived by the transit time distribution approach and the ratio of the anthropogenic δ¹³C/dissolved inorganic carbon change (RC). A significant C_antincrease (up to 45 μmol kg⁻¹) and δ¹³C decrease (up to −0.9‰) extends to ~2,000 m in the Canada and Makarov Basin. We find distinctly different RC values for the intermediate water (300–2,000 m) and upper halocline water (<200 m) of −0.020 and −0.012‰ (μmol kg⁻¹)⁻¹, respectively, which identifies two sources of C_antaccumulation from North Atlantic and North Pacific. Furthermore, estimated RC for intermediate waters is the same as the RC observed in the Greenland Sea and the rate of anthropogenic dissolved inorganic carbon increase estimated for intermediate waters at 0.9 μmol kg⁻¹yr⁻¹is identical to the estimated rate in the Iceland Sea. These observations indicate that the high rate of C_antaccumulation and δ¹³C decrease in the Arctic Ocean is primarily a result of the input of C_ant, via ventilation of intermediate waters, from the Nordic Sea rather than local anthropogenic CO₂uptake within the Arctic Basin. We determine the preindustrial δ¹³C (δ¹³C_PI) distributions and find distinct δ¹³C_PIsignatures of the intermediate and upper halocline waters that reflect the difference in δ¹³C_PI–PO₄relationship of Atlantic and Pacific source water.

    

   more » « less
4. Size‐Fractionated Compositions of Marine Suspended Particles in the Western Arctic Ocean: Lateral and Vertical Sources

   https://doi.org/10.1029/2020JC016144  

   Xiang, Yang ; Lam, Phoebe J. ( August 2020 , Journal of Geophysical Research: Oceans)

   We present full water depth sections of size‐fractionated (1–51 μm; >51 μm) concentrations of suspended particulate matter and major particle phase composition (particulate organic matter [POM], including its carbon isotopic composition [POC‐δ¹³C] and C:N ratio, calcium carbonate [CaCO₃], opal, lithogenic particles, and iron and manganese [oxyhydr]oxides) from the U.S. GEOTRACES Arctic Cruise (GN01) in the western Arctic in 2015. Whereas biogenic particles (POM and opal) dominate the upper 1,000 m, lithogenic particles are the most abundant particle type at depth. Minor phases such as manganese (Mn) oxides are higher in GN01 than in any other U.S. GEOTRACES cruises so far. Extremely depleted POC‐δ¹³C, as low as ~ −32‰, is ubiquitous at the surface of the western Arctic Ocean as a result of different growth rates of phytoplankton. Moderate penetration of depleted POC‐δ¹³C to depth indicates active sinking of large particles in the central basin. Lateral transport from the Chukchi shelf is also of significance in the western Arctic, as is evident from increases in biogenic silica to POC ratios and Mn oxide concentrations in the halocline, as well as lithogenic element contents in the deep waters. Our study supports previous suggestions of the near absence of CaCO₃in the Arctic Basin. This study presents the first data set of concentration and composition of suspended particles in the western Arctic Ocean and sheds new light on the vertical and lateral processes that govern particle distribution in this enclosed ocean basin.

    

   more » « less
5. Estimating Mass Flux From Size‐Fractionated Filtered Particles: Insights Into Controls on Sinking Velocities and Mass Fluxes in Recent U.S. GEOTRACES Cruises

   https://doi.org/10.1029/2021GB007292  

   Xiang, Yang ; Lam, Phoebe J. ; Burd, Adrian B. ; Hayes, Christopher T. ( April 2022 , Global Biogeochemical Cycles)

   We compile full ocean‐depth size‐fractionated (1–51 and >51 μm) particle concentration and composition of suspended particulate matter from three recent U.S. GEOTRACES cruises, and exploit detailed information of particle characteristics measured to give insights into controls on sinking velocity and mass flux. Our model integrates the concept of fractal scaling into Stokes' Law by incorporating one of two porosity‐size power law relationships that result in fractal dimensions of 1.4 and 2.1. The medians of pump‐derived total (>1 μm) mass flux in the upper 100 m of gyre stations are 285.1, 609.2, and 99.3 mg/m²/d in the North Atlantic, Eastern Tropical South Pacific, and Western Arctic Ocean cruises, respectively. In this data set, variations in particle concentration were generally more important than sinking velocity in controlling variations in mass flux. We examine different terms in a Stokes' Law model to explore how variations in particle and water column characteristics from these three cruises affect mass flux. The decomposition of different aspects of the Stokes' relationship sheds light on the lowest total mass flux of the three cruises in the Western Arctic, which could be explained by the Arctic having the lowest particle concentrations as well as the lowest sinking velocities due to having the smallest particle sizes and the most viscous water. This work shows the importance of both particle characteristics and size distribution for mass fluxes, and similar methods can be applied to existing and future size‐fractionated filtered particulate measurements to improve our understanding of the biological pump elsewhere.

    

   more » « less