Phytoplankton productivity in the Bransfield strait recorded in isotopes of sedimentary chlorophyll-a
Aymeric P. M. Servettaz, Yuta Isaji, Nanako O. Ogawa, Chisato Yoshikawa, Francisco J. Jiménez-Espejo, Naohiko Ohkouchi
The Southern Ocean is an ecologically active zone with massive phytoplankton blooms in spring and summer, supported by nutrient-rich waters supplied by deep-water upwelling. Variations in sea-ice extent and seasonality influence the surface water nutrient and light availability and thus the phytoplanktonic productivity. In this study, we characterize primary productivity in the Antarctic peninsula region during the past 2000 years by analysing the chloropigments preserved in a sediment core collected in the Bransfield Strait (61.99°S, 55.09°W), and discuss possible environmental changes. We measured the concentration of chlorophyll-a and its main derivatives as well as their carbon and nitrogen isotopic compositions, which reflect the physiology and ecology of the phytoplankton and the surface water chemistry.
The d13C values of chlorophyll-a (d13Cchla) fluctuate substantially throughout the core (from -21‰ to -16‰), and are correlated positively (r2 = 0.49, p_value < 0.01) with chlorophyll-a concentrations. Rapid growth during phytoplankton bloom can primarily explain both the 13C-enrichment in chlorophyll and a fast sinking followed by deposition that results in enhanced preservation of organic matter. The d15N of chlorophyll-a (d15Nchla) ranges from -6.0‰ to -3.2‰, and is expected to reflect the d15N of the phytoplankton cells with an offset of -4.8‰ (Ohkouchi et al., 2006). d15N indicates a partial consumption of surface water nitrate, as similarly observed in the modern Southern Ocean. The +1‰ increase in d15N in the second half of the record may indicate enhanced nitrate consumption without reaching complete depletion, because of higher productivity and/or reduced nitrate pool. These results constitute the first chlorophyll-specific isotopic analysis in a paleoclimate record, providing unprecedented insights on the past primary productivity of the Southern Ocean.
Snowfall warming and intermittency: biases and variability of temperature during snowfall in Antarctica
(titre provisoire)
Aymeric P. M. Servettaz, Cécile Agosta, Christoph Kittel, Anaïs J. Orsi
Antarctica, the coldest and driest continent, is home to the largest ice sheet, whose mass is predominantly recharged by snowfall. A common feature of polar regions is the warming associated with snowfall, as moist oceanic air and cloud cover increase the surface temperature. Consequently, snowfall accumulated onto the ice sheet is deposited under unusually warm conditions. Here we use a polar-oriented regional atmospheric model to study the statistical difference between average and snowfall-weighted temperatures. Most of Antarctica experiences a warming scaling with snowfall, with strongest warming at low accumulation sites. Heavier snowfall in winter contributes to cool the snowfall-weighted temperature, but this effect is overwritten by the event-scale warming associated with precipitating atmospheric systems, which particularly contrast with the extremely cold conditions in winter. Consequently, the seasonal range of snowfall-weighted temperature is reduced by 20%. On the other hand, annual snowfall-weighted temperature shows 80% more interannual variability than annual temperature, due to irregularity of precipitations. Disturbance in apparent annual temperature cycle and interannual variability may have major implications for water isotopes, which are deposited with snowfall and commonly used for paleo-temperature reconstructions.
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