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.
Highlights
Reduced surface–deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2
Deep mixing during fall and winter allowed for surface-ocean refueling with nutrients from a potentially enriched deep reservoir, which generated a carbon sink in the glacial sea-ice zone (SIZ)
We rule out seasonal effects, because sediment trap studies show that the diatom and radiolarian export in the SIZ of the Southern Ocean (SO) occur synchronously and are restricted to spring–summer[26]
Summary
Reduced surface–deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. Glacial– interglacial changes in surface ocean stratification, which control ocean atmosphere exchange and the availability of nutrients, have been discussed contentiously This has resulted in different notions of the impact of physical and biological processes in ice-free and ice-covered areas on the glacial–interglacial climate evolution[3,5,13]. Isotope records of diatom-bound nitrogen (d15N) are interpreted to indicate a low-productivity glacial seasonal sea-ice zone (SIZ) resulting from constricted nutrient supply to the surface ocean, owing to permanent and enhanced nearsurface stratification[3,13]. In combination with the diatom isotope data, these signals provide an enhanced framework to detect changes of upper and lower water column conditions, and the pattern and glacial–interglacial variability of stratification and nutrient exchange. Deep mixing during fall and winter allowed for surface-ocean refueling with nutrients from a potentially enriched deep reservoir, which generated a carbon sink in the glacial SIZ
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