Cobalt scavenging in the mesopelagic ocean and its influence on global mass balance: Synthesizing water column and sedimentary fluxes

Abstract

Cobalt scavenging in the mesopelagic ocean and its influence on global mass balance: Synthesizing water column and sedimentary fluxes. AbstractIn the ocean, dissolved cobalt is affected by both nutrient cycling and scavenging onto manganese oxides. The latter process concentrates Co in pelagic sediments, resulting in a small deep water inventory. While the flux of scavenged cobalt to sediments appears steady on timescales greater than 100,000 years, its residence time in the water column is short, approximately 130 years. Using results from recent GEOTRACES expeditions, we show net removal of dissolved Co from the deep ocean on the order of 0.043 pM year -1 , which corresponds to a turnover time of 980 years. Scavenging in deep ocean water masses is too slow to match cobalt accumulation rates in marine sediments, requiring most of the scavenging flux to derive from the mesopelagic ocean (< 1500 m depth) where nutrient cycling is active. Based on differences between the Co:P stoichiometry in particles sinking from the euphotic zone and dissolved Co:P remineralization ratios, we calculate areal scavenging rates in the North Atlantic and South Pacific basins on the order of 1.5 and 0.7 μmol m -2 year -1 , respectively, which agree with long-term accumulation rates in Atlantic and Pacific sediments. In both basins, over 50% of the scavenged flux of cobalt occurs in the upper 500 m, resulting in decadal turnover times in the mesopelagic. An assessment of sources suggests that the marine cobalt cycle is approximately in balance, but that this inventory may be sensitive to long term trends in the intensity of oxygen minimum zones, which account for ~25% of the annual cobalt source to the modern oceans.

Replacement Times of a Spectrum of Elements in the North Atlantic Based on Thorium Supply

“…The North Atlantic replacement time for Co in this analysis (140 ± 70 years) is similar to the best previous residence time estimates (Hawco et al, ; Saito & Moffett, ; Tagliabue et al, ) Additionally, Al has a deep ocean replacement time of 50 ± 10 years, again broadly consistent with previous residence time estimates (Bruland et al, ; Chester & Jickells, ) and being slightly longer than the residence time of Th, also expected based on previous work (e.g., Moran & Moore, ). The mid‐ocean ridge in the vicinity of GA03 may be a source of Al, either from hydrothermal input (Measures et al, ) or sediment dissolution on the flanks of the ridge (Middag et al, ), and either source would contribute to our replacement time overestimating the true residence time here.…”

“…The Bruland et al () method produced a similar residence time estimate as Little et al () for Cd of 22,000–45,000 years. Finally, Roshan et al () also calculated a shorter Zn residence time (3,000 ± 600 years) than Little et al (), by adding recently observed hydrothermal sources of Zn to the sum of Zn sources compiled by Little et al In contrast to these relatively long time scales, for Mn, Co, and Al, the best available estimates are much shorter than the time scale of deep ocean circulation (20–40 years [Bruland et al, ], 40–130 years [Hawco et al, ; Saito & Moffett, ], and 45–90 years [Bruland et al, ], respectively). These estimates are based on scavenging and particle sedimentation, likely the major removal mechanism for these elements.…”

The Role of External Inputs and Internal Cycling in Shaping the Global Ocean Cobalt Distribution: Insights From the First Cobalt Biogeochemical Model

“…Dust, sediments, and rivers supply 6.5 × 10 7 , 6.8 × 10 8 , and 5.7 × 10 6 mol of Co annually. The sediment source compares favorably to an independent estimate (~6 × 10 8 mol of Co annually) based on simpler calculations from field data sets (Hawco et al, ). Primary production consumes 23.9 × 10 8 mol of Co, with much of this dCo sink balanced by recycling of 20.9 × 10 8 mol from zooplankton, while regeneration of particulate organic Co resupplies a further 8.6 × 10 8 mol each year.…”

Competitive inhibition of cobalt uptake by zinc and manganese in a pacificProchlorococcusstrain: Insights into metal homeostasis in a streamlined oligotrophic cyanobacterium

“…(Sánchez‐Baracaldo ; Braakman et al ). Despite rising atmospheric O 2 levels during this time, the oceans were not uniformly oxygenated (Sperling et al ), which probably maintained large cobalt sources from anoxic sediments, as observed in the modern ocean (Hawco et al ). Concurrent evolution toward greater use of zinc in eukaryotic metabolism and expansion of eukaryotic phytoplankton into the open ocean probably led to increased zinc uptake and maintained relatively low Zn′ in the oligotrophic gyres (Dupont et al ).…”

“…Complexation by intracellular thiols drives the extremely low abundance of Cu and Zn ions in cyanobacterial cytoplasm, creating the expectation that Cu′ and Zn′ were low prior to the Great Oxidation Event (Saito et al ; Waldron and Robinson ). Importantly, the cobalt content of the anoxic, ferruginous ocean was probably higher without oxidative scavenging processes that characterize the modern cobalt cycle (Saito et al ; Swanner et al ; Hawco et al ). Indeed, the cobalt plumes in modern oxygen minimum zones hint of a much greater cobalt inventory when the entire ocean was anoxic (Noble et al , 2017; Hawco et al ), providing a basis for the evolution of cobalt‐dependent metabolism (e.g., vitamin B 12 ) despite the modern scarcity of cobalt.…”