skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00 PM ET on Thursday, June 13 until 2:00 AM ET on Friday, June 14 due to maintenance. We apologize for the inconvenience.

Title: Global Contrasts Between Oceanic Cycling of Cadmium and Phosphate

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.

more » « less
Award ID(s):
Author(s) / Creator(s):
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Global Biogeochemical Cycles
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    In hyperalkaline () fluids that have participated in low‐temperature (<150) serpentinization reactions, the dominant form of C is often methane (), but the origin of thisis uncertain. To assessorigin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic profiles of fluid‐hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue (D and) relative abundances less than equilibrium, consistent with substantial microbialproduction. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations andacross fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source ofis evidenced by the presence of‐bearing fluid inclusions in the Samail Ophiolite and our measurement of highvalues of ethane and propane, which are similar to those reported in studies of‐rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methanotrophs, indicating that microbial consumption ofin the ophiolite may further enrichin13C. We conclude that substantial microbialis produced under varying degrees of C limitation and mixes with abioticreleased from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.

    more » « less
  2. Abstract

    The air‐sea exchange of oxygen (O2) is driven by changes in solubility, biological activity, and circulation. The total air‐sea exchange of O2has been shown to be closely related to the air‐sea exchange of heat on seasonal timescales, with the ratio of the seasonal flux of O2to heat varying with latitude, being higher in the extratropics and lower in the subtropics. This O2/heat ratio is both a fundamental biogeochemical property of air‐sea exchange and a convenient metric for testing earth system models. Current estimates of the O2/heat flux ratio rely on sparse observations of dissolved O2, leaving it fairly unconstrained. From a model ensemble we show that the ratio of the seasonal amplitude of two atmospheric tracers, atmospheric potential oxygen (APO) and the argon‐to‐nitrogen ratio (Ar/O2), exhibits a close relationship to the O2/heat ratio of the extratropics (40–). The amplitude ratio,/, is relatively constant within the extratropics of each hemisphere due to the zonal mixing of the atmosphere./is not sensitive to atmospheric transport, as most of the observed spatial variability in the seasonal amplitude ofAPO is compensated by similar variations in(Ar/). From the relationship between/heat and/in the model ensemble, we determine that the atmospheric observations suggest hemispherically distinct/heat flux ratios of 3.30.3 and 4.70.8 nmolbetween 40 andin the Northern and Southern Hemispheres respectively, providing a useful constraint forand heat air‐sea fluxes in earth system models and observation‐based data products.

    more » « less
  3. Abstract

    In this study, we report on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan (USA), highlighting stratified, convective, and transitional mixing periods. Measurements were collected using a combination of moored instruments and microstructure profiles. Observations during the stratified summer showed a shallow, wind‐driven surface mixed layer (SML) with locally elevated dissipation rates in the thermocline () potentially associated with internal wave shear. Below the thermocline, turbulence was weak () and buoyancy‐suppressed (< 8.5), with low hypolimnetic mixing rates () limiting benthic particle delivery. During the convective winter period, a diurnal cycle of radiative convection was observed over each day of measurement, where temperature overturns were directly correlated with elevated turbulence levels throughout the water column (;). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density (TMD3.98) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period (), hypolimnetic velocity shear was overwhelmed by weakly stable stratification (;), limiting the development of the SML. These results highlight the importance of radiative convection for breaking down weak hypolimnetic stratification and driving energetic, full water column mixing during a substantial portion of the year (>100 days at our sample site). Ongoing surface water warming in the Laurentian Great Lakes is significantly reducing the annual impact of convective mixing, with important consequences for nutrient cycling, primary production, and benthic‐pelagic coupling.

    more » « less
  4. Abstract

    Estimates of turbulence kinetic energy (TKE) dissipation rate (ε) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates ofεwith concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates ofεat a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scaleand the newer Langmuir stability length scale, suggesting that ocean‐side friction velocityimplicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes driftandthat is likely facilitated by the steady Southeast trade winds regime. In certain regimes,, whereis the von Kármán constant andis instrument depth, and surface buoyancy flux capture our estimates ofwell, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based onand, scalesεwell at times but is overall less consistent than. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes.

    more » « less
  5. Abstract

    A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH+ OCHNH/ CHNH + HO, CHNH+ HCH+ NH, CHNHCHNH, and CHNH + OCHNH + HOwere calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNHand its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NHgroup in CHNHcompared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NHin methylamine oxidation.

    more » « less