Stable oxygen and carbon isotopes in planktic foraminifers from Arctic Ocean surface sediments: Reflection of the low salinity surface water layer

Abstract

Planktic foraminifers Neogloboquadrina pachyderma (sin.) from 87 eastern and central Arctic Ocean surface sediment samples were analyzed for stable oxygen and carbon isotope composition. Additional results from 52 stations were taken from the literature. The lateral distribution of δ 18O ( 18 O 16 O ) values in the Arctic Ocean reveals a pattern of roughly parallel, W-E stretching zones in the Eurasian Basin, each ∼0.5‰ wide on the δ 18O scale. The low horizontal and vertical temperature variability in the Arctic halocline waters (0–100 m) suggests only little influence of temperature on the oxygen isotope distribution of N. pachyderma (sin.). The zone of maximum δ 18O values of up to 3.8‰ is situated in the southern Nansen Basin and relates to the tongue of saline (> 33%.) Atlantic waters entering the Arctic Ocean through the Fram Strait. δ 18O values decrease both to the Barents Shelf and to the North Pole, in accordance with the decreasing salinities of the halocline waters. In the Nansen Basin, a strong N-S δ 18O gradient is in contrast with a relatively low salinity change and suggests contributions from different freshwater sources, i.e. salinity reduction from sea ice meltwater in the south and from light isotope waters (meteoric precipitation and river-runoff) in the northern part of the basin. North of the Gakkel Ridge, δ 18O and salinity gradients are in good accordance and suggest less influence of sea ice melting processes. The δ 13C ( 13 C 12 C ) values of N. pachyderma (sin.) from Arctic Ocean surface sediment samples are generally high (0.75–0.95‰). Lower values in the southern Eurasian Basin appear to be related to the intrusion of Atlantic waters. The high δ 13C values are evidence for well ventilated surface waters. Because the perennial Arctic sea ice cover largely prevents atmosphere-ocean gas exchange, ventilation on the seasonally open shelves must be of major importance. Lack of δ 13C gradients along the main routes of the ice drift from the Siberian shelves to the Fram Strait suggests that primary production (i.e. CO 2 consumption) does probably not change the CO 2 budget of the Arctic Ocean significantly.