Benthic foraminiferal abundance and stable isotope changes in the Indian Ocean sector of the Southern Ocean during the last 20 kyr: Paleoceanographic implications

Abstract

Benthic foraminiferal faunas and benthic stable isotopes during the last 20 kyr were investigated in core MD02-2488 (46°28.8′S, 88°01.3′E, 3420 m water depth) from the southeast Indian Ocean sector of the Southern Ocean in order to document paleoceanographic changes from the last glacial, through deglaciation and into the Holocene. Variations occurred in the accumulation rates, species composition, diversity, and stable isotopes during the last 20 kyr. The benthic δ 18O and δ 13C records during the last ~ 28 kyr show that no significant deep water circulation changes took place before ~ 15.6 calendar kyr (ka). The δ 13C increase at 15.6 ka is interpreted as reflecting the arrival of better ventilated deep waters at the site and is associated with the progressive increase in % of miliolids from ~ 15 ka to 0 ka, an interval (~ 16–15 ka) of low benthic foraminiferal accumulation rates (BFARs) and an increase in the relative abundance of Stainforthia complanata. Deep waters were ~ 2.2 ± 0.4 °C colder during the last glacial than at present. Carbonate dissolution proxies indicate that carbonate dissolution was higher during the last glacial than during the last deglacial and Holocene. BFARs and the percentage of species indicative of high-productivity suggest that the flux of organic matter resulting from surface water productivity was generally higher between ~ 20 and 13 ka than between ~ 12 and 0 ka, consistent with the observed higher proportion of biosiliceous sediment at ~ 20–14 ka and published records. The combined percentages of the two most abundant species, Alabaminella weddellensis and Epistominella exigua (‘phytodetritus species’), suggest that seasonality of productivity (intermittency of primary productivity) fluctuated over the last 20 kyr and was higher during the last glacial and deglaciation (20–10 ka) compared with the Holocene (10–0 ka). The relative abundances of A. weddellensis and E. exigua do not covary over the last 20 kyr, an observation also reported from other areas.