Polar sea-ice is a unique habitat for virus, bacteria, microalgae and zooplankton, which are enclosed in an ice matrix at low temperature (down to - 20°C) and low light levels, where the only liquids are high salinity brines (up to 82 PSU) in channels and pockets. Survival in these conditions requires a complex series of physiological and metabolic adaptations, but organisms thrive in the sea-ice, and their prolific growth play a fundamental role in polar ecosystems and carbon cycle. The planktic foraminifer Neogloboquadrina pachyderma (Ehrenberg, 1861) is the only taxon among sea-ice zooplankton to secrete a calcareous test (= shell). At present, N. pachyderma specimens are abundant in the Antarctic sea-ice, but very rare in the Arctic sea-ice. They have likely adopted different test-building and biocalcification strategies depending on whether they grow in the seawater column or sea-ice. Recent culture experiments with juvenile and pre-adult N. pachyderma collected from sea-ice during the austral winter in the Antarctic Weddell Sea (ANT-XXIX/6 of RV Polarstern in 2013) have shown that they can actually biocalcify in extreme salinity conditions mimicking those of the sea-ice brines. Trace element analyses on these cultured N. pachyderma specimens, reveal increased values of test Mg/Ca, Sr/Ca and Na/Ca ratios at higher salinities and stress the strong influence of high salinity brines on the Mg/Ca paleothermometer. Consequently, the temperatures of this proxy may be overestimated. We analyzed specimens from the upper and lower layers of sea-ice cores collected during the austral winter in the Weddell Sea (ANT-XXIX/7 of RV Polarstern in 2013) and from seawater column sampled during a subsequent cruise during the austral autumn (Polarstern PS112 in 2018). We performed NanoSIMS (MNHN, Paris) intra-test mapping with nanoscale lateral resolution (< 100 nm) of Ca, Mg, Na, K and Sr following the sub-micrometric growth structure of the test’s chambers, characterized by an alternation of organic and calcitic layers. We found a heterogeneous distribution of Mg/Ca, Na/Ca, K/Ca, Sr/Ca ratios within the test, characterized by alternating enriched and depleted bands parallel to the test surface. The Na and K banding are highly correlated, whereas there is no obvious correlation between the banding of the other trace elements. Na and K enrichments are observed around thin organic layers interspersed with calcitic layers, associated with the sequential chamber formation of the test. Mg enrichments are observed both around the organic layers between the calcitic layers and within the calcitic layers, the latter being possibly related to the continuous thickening of the calcitic layers once formed. In addition to the impact of high salinity, the intra-test distribution of Mg in the analyzed individuals also shows a significant biological control on Mg incorporation during test construction. Comparison of intra-test distribution patterns of Mg/Ca, Sr/Ca, K/Ca and Na/Ca between specimens collected from sea- ice and the seawater column confirms that N. pachyderma appears to be able to calcify in Antarctic sea-ice within brine pockets and channels, and that their test has a characteristic geochemical sea-ice signature. Furthermore, the comparison of Mg/Ca, Na/Ca, K/Ca and Sr/Ca ratios between the last and first chambers may provide new insights into the life cycle of this fascinating species of planktic foraminifera that thrives in polar environments, including sea-ice. A better understanding of the incorporation of these key trace elements in N. pachyderma tests taken from sea-ice may help to use them as multi- proxies of sea-ice palaeoenvironments and their evolution in the past.
Looking for a geochemical imprinting of sea-ice environment in the planktic foraminiferal Neogloboquadrina pachyderma
Morigi C.;
2023-01-01
Abstract
Polar sea-ice is a unique habitat for virus, bacteria, microalgae and zooplankton, which are enclosed in an ice matrix at low temperature (down to - 20°C) and low light levels, where the only liquids are high salinity brines (up to 82 PSU) in channels and pockets. Survival in these conditions requires a complex series of physiological and metabolic adaptations, but organisms thrive in the sea-ice, and their prolific growth play a fundamental role in polar ecosystems and carbon cycle. The planktic foraminifer Neogloboquadrina pachyderma (Ehrenberg, 1861) is the only taxon among sea-ice zooplankton to secrete a calcareous test (= shell). At present, N. pachyderma specimens are abundant in the Antarctic sea-ice, but very rare in the Arctic sea-ice. They have likely adopted different test-building and biocalcification strategies depending on whether they grow in the seawater column or sea-ice. Recent culture experiments with juvenile and pre-adult N. pachyderma collected from sea-ice during the austral winter in the Antarctic Weddell Sea (ANT-XXIX/6 of RV Polarstern in 2013) have shown that they can actually biocalcify in extreme salinity conditions mimicking those of the sea-ice brines. Trace element analyses on these cultured N. pachyderma specimens, reveal increased values of test Mg/Ca, Sr/Ca and Na/Ca ratios at higher salinities and stress the strong influence of high salinity brines on the Mg/Ca paleothermometer. Consequently, the temperatures of this proxy may be overestimated. We analyzed specimens from the upper and lower layers of sea-ice cores collected during the austral winter in the Weddell Sea (ANT-XXIX/7 of RV Polarstern in 2013) and from seawater column sampled during a subsequent cruise during the austral autumn (Polarstern PS112 in 2018). We performed NanoSIMS (MNHN, Paris) intra-test mapping with nanoscale lateral resolution (< 100 nm) of Ca, Mg, Na, K and Sr following the sub-micrometric growth structure of the test’s chambers, characterized by an alternation of organic and calcitic layers. We found a heterogeneous distribution of Mg/Ca, Na/Ca, K/Ca, Sr/Ca ratios within the test, characterized by alternating enriched and depleted bands parallel to the test surface. The Na and K banding are highly correlated, whereas there is no obvious correlation between the banding of the other trace elements. Na and K enrichments are observed around thin organic layers interspersed with calcitic layers, associated with the sequential chamber formation of the test. Mg enrichments are observed both around the organic layers between the calcitic layers and within the calcitic layers, the latter being possibly related to the continuous thickening of the calcitic layers once formed. In addition to the impact of high salinity, the intra-test distribution of Mg in the analyzed individuals also shows a significant biological control on Mg incorporation during test construction. Comparison of intra-test distribution patterns of Mg/Ca, Sr/Ca, K/Ca and Na/Ca between specimens collected from sea- ice and the seawater column confirms that N. pachyderma appears to be able to calcify in Antarctic sea-ice within brine pockets and channels, and that their test has a characteristic geochemical sea-ice signature. Furthermore, the comparison of Mg/Ca, Na/Ca, K/Ca and Sr/Ca ratios between the last and first chambers may provide new insights into the life cycle of this fascinating species of planktic foraminifera that thrives in polar environments, including sea-ice. A better understanding of the incorporation of these key trace elements in N. pachyderma tests taken from sea-ice may help to use them as multi- proxies of sea-ice palaeoenvironments and their evolution in the past.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
