Search

The Atlantic sector of the Southern Ocean is the world's main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses—ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4–9.7 Sv) yielding larger rates than tracer-based estimates (3.7–4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m - 3 γ n is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models. [ABSTRACT FROM AUTHOR]

Cycling of deepwater silicon (Si) within the Southern Ocean, and its transport into other ocean basins, may be an important player in the uptake of atmospheric carbon, and global climate. Recent work has shown that the Si isotope (denoted by δ29Si or δ30Si) composition of deep sea sponges reflects the availability of dissolved Si during growth, and is a potential proxy for past deep and intermediate water silicic acid concentrations. As with any geochemical tool, it is essential to ensure analytical precision and accuracy, and consistency between methodologies and laboratories. Analytical bias may exist between laboratories, and sponge material may have matrix effects leading to offsets between samples and standards. Here, we report an interlaboratory evaluation of Si isotopes in Antarctic and sub-Antarctic sponges. We review independent methods for measuring Si isotopes in sponge spicules. Our results show that separate subsamples of non-homogenized sponges measured by three methods yield isotopic values within analytical error for over 80% of specimens. The relationship between δ29Si and δ30Si in sponges is consistent with kinetic fractionation during biomineralization. Sponge Si isotope analyses show potential as palaeoceaongraphic archives, and we suggest Southern Ocean sponge material would form a useful additional reference standard for future spicule analyses. [ABSTRACT FROM AUTHOR]

Full-depth measurements of δ18O from 2008 to 2010 enclosing the Weddell Gyre in the Southern Ocean are used to investigate the regional freshwater budget. Using complementary salinity, nutrients and oxygen data, a four-component mass balance was applied to quantify the relative contributions of meteoric water (precipitation/glacial input), sea-ice melt and saline (oceanic) sources. Combination of freshwater fractions with velocity fields derived from a box inverse analysis enabled the estimation of gyre-scale budgets of both freshwater types, with deep water exports found to dominate the budget. Surface net sea-ice melt and meteoric contributions reach 1.8% and 3.2%, respectively, influenced by the summer sampling period, and -1.7% and +1.7% at depth, indicative of a dominance of sea-ice production over melt and a sizable contribution of shelf waters to deep water mass formation. A net meteoric water export of approximately 37 mSv is determined, commensurate with local estimates of ice sheet outflow and precipitation, and the Weddell Gyre is estimated to be a region of net sea-ice production. These results constitute the first synoptic benchmarking of sea-ice and meteoric exports from the Weddell Gyre, against which future change associated with an accelerating hydrological cycle, ocean climate change and evolving Antarctic glacial mass balance can be determined. [ABSTRACT FROM AUTHOR]

Abstract: Despite a growing body of work that uses diatom δ 30Si to reconstruct past changes in silicic acid utilisation, few studies have focused on calibrating core top data with modern oceanographic conditions. In this study, a microfiltration technique is used to divide Southern Ocean core top silica into narrow size ranges, separating components such as radiolaria, sponge spicules and clay minerals from diatoms. Silicon isotope analysis of these components demonstrates that inclusion of small amounts of non-diatom material can significantly offset the measured from the true diatom δ 30Si. Once the correct size fraction is selected (generally 2–20μm), diatom δ 30Si shows a strong negative correlation with surface water silicic acid concentration (R 2 =0.92), highly supportive of the qualitative use of diatom δ 30Si as a proxy for silicic acid utilisation. The core top diatom δ 30Si matches well with mixed layer filtered diatom δ 30Si from published in situ studies, suggesting little to no effect of either dissolution on export through the water column, or early diagenesis, on diatom δ 30Si in sediments from the Southern Ocean. However, the core top diatom δ 30Si shows a poor fit to simple Rayleigh or steady state models of the Southern Ocean when a single source term is used. The data can instead be described by these models only when variations in the initial conditions of upwelled silicic acid concentration and δ 30Si are taken into account, a caveat which may introduce some error into quantitative reconstructions of past silicic acid utilisation from diatom δ 30Si. [Copyright &y& Elsevier]

Abstract: The Crozet Plateau is an area of interest as it is a region of the Southern Ocean where annual primary productivity is relatively high. We describe spatial and temporal variations in the biomarker distributions and carbon and nitrogen isotopic signatures of organic matter formed within surface waters around the plateau during an algal bloom. In the centre of the bloom, diatom-derived sterols dominate, whereas alkenones are relatively more abundant outside and at the edge of the bloom. Bulk organic δ13C and δ15N values are enriched inside the bloom area, ranging from −18.7‰ to −17.6‰ and +1.6‰ to +3.7‰, respectively, compared to −25.7‰ to −20.9‰ and −2.0‰ to +1.3‰, respectively, outside the bloom area. Variation in bulk δ15N values is attributed primarily to NH3 or dissolved organic N uptake, with only a minor contribution from Rayleigh fractionation of the pool. Compound specific δ13C values (cholest-5-en-3β-ol, cholesta-5,22-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol, 24-methylcholesta-24(28)-dien-3β-ol and alkenones) are also enriched in the Fe fertilised region relative to other areas, presumably reflecting higher phytoplankton growth rate, consumption of dissolved inorganic carbon and perhaps more intense use of carbon concentrating mechanisms. The observations confirm previous interpretations that Fe fertilisation in the Crozet region has affected nutrient utilisation and the algal community structure; it also suggests that lipid biomarkers and their δ13C values record a specific bloom signature that may be recorded in underlying sediments. [Copyright &y& Elsevier]