The seasonal sea-ice zone in the glacial Southern Ocean as a carbon sink

Abstract

Reduced surface–deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. However, identification of the biological and physical conditions involved and the related processes remains incomplete. Here we specify Southern Ocean surface–subsurface contrasts using a new tool, the combined oxygen and silicon isotope measurement of diatom and radiolarian opal, in combination with numerical simulations. Our data do not indicate a permanent glacial halocline related to melt water from icebergs. Corroborated by numerical simulations, we find that glacial surface stratification was variable and linked to seasonal sea-ice changes. During glacial spring–summer, the mixed layer was relatively shallow, while deeper mixing occurred during fall–winter, allowing for surface-ocean refueling with nutrients from the deep reservoir, which was potentially richer in nutrients than today. This generated specific carbon and opal export regimes turning the glacial seasonal sea-ice zone into a carbon sink.

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…Contamination, via the presence of residual tephras or clays can compromise the precision of reconstructions, with the introduction of significant isotopic offsets (Morley et al, 2005;Lamb et al, 2007;Brewer et al, 2008). This is often more pronounced for diatoms (and radiolarians; Abelmann et al, 2015) due to the ability for contaminant valve adherence and occlusion, over sponge spicules that can be picked. Clay δ 30 Si compositions are estimated between −2.95‰ and +2.5 (Douthitt, 1982;Georg et al, 2009;Opfergelt and Delmelle, 2012), which can significantly lower reported δ 30 Si diatom values [published values range between −0.07 and +3.05‰; (De La Rocha et al, 2000;Cardinal et al, 2007;Sun et al, 2013;Panizzo et al, 2014Panizzo et al, , 2016] and outside of analytical uncertainty, while high sample presence of different phases of bSiO 2 can have a similar effect (e.g., δ 30 Si sponge signatures vary between −5.72 and +0.…”

“…A recent example, used both oxygen and Si isotopes in diatoms and radiolarians, from glacial-aged sediments from the Southern Ocean, to provide reconstructions of seasonal nutrient cycling. Data show strong variability in the mixed layer depth in the seasonal sea-ice zone, which allowed for sufficient nutrient exchange between surface and deeper waters, thereby fueling carbon drawdown in an otherwise highly stratified glacial Southern Ocean (Abelmann et al, 2015). Diatom δ 30 Si and organic-bound δ 15 N archives from the Southern Ocean have also been coupled with a spicule δ 30 Si record of bottom water DSi concentrations to investigate DSi utilization across the last glacial termination.…”

“…Despite an extensive effort by many researchers over the past five decades (e.g., Mopper and Garlick, 1971;Labeyrie, 1979;Labeyrie and Juillet, 1982;Chung-Ho and Hsueh-Wen, 1985;Leclerc and Labeyrie, 1987;Matheney and Knauth, 1989;Shemesh et al, 1992;Swann and Leng, 2009;Pike et al, 2013;Crespin et al, 2014;Abelmann et al, 2015), uncertainty associated with the bSiO 2 -water oxygen isotope fractionation has limited the application of bSiO 2 δ 18 O values as a palaeoceanographic proxy. The disparate bSiO 2 -water fractionation factors have been attributed to two primary causes: (1) methodological biased and/or incomplete removal of hydroxyl oxygen, and (2) potential alteration of δ 18 O values during bSiO 2 formation/diagenetic alteration of δ 18 O values on geologic timescales.…”

“…However, there have been a small number of studies that have attempted to constrain a fractionation factor for Si isotopes in radiolarians, which have estimated a similar fractionation range as for modern diatoms (ca. −0.5 to −2.1‰; Abelmann et al, 2015). As such, there is potential for radiolarian δ 30 Si to be used, in combination with modeling efforts, to constrain mid-depth to surface DSi systematics.…”

Competition between Silicifiers and Non-silicifiers in the Past and Present Ocean and Its Evolutionary Impacts

“…Silicon transport into and within the cell is likely the basis for the fractionation of stable Si isotopes. All silicifiers investigated to date fractionate Si isotopes relative to the seawater in which they grow, but sponges have a more variable and potentially greater isotopic fractionation than either diatoms or radiolarians (de la Rocha et al, 1997;Hendry and Robinson, 2012;Hendry et al, 2014;Abelmann et al, 2015). Such isotopic fractionation may be related to the biochemical pathways involved in Si metabolism, and may reflect the organism's affinity for silicic acid and efficiency of uptake and utilization.…”