The impact of African aridity on the isotopic signature of Atlantic deep waters across the Middle Pleistocene Transition

Abstract

A high resolution analysis of benthic foraminifera as well as of aeolian terrigenous proxies extracted from a 37 m-long marine core located off the Mauritanian margin spanning the last ~ 1.2 Ma, documents the possible link between major continental environmental changes with a shift in the isotopic signature of deep waters around 1.0–0.9 Ma, within the so-called Mid-Pleistocene Transition (MPT) time period. The increase in the oxygen isotopic composition of deep waters, as seen through the benthic foraminifera δ18O values, is consistent with the growth of larger ice sheets known to have occurred during this transition. Deep-water mass δ13C changes, also estimated from benthic foraminifera, show a strong depletion for the same time interval. This drastic change in δ13C values is concomitant with a worldwide 0.3‰ decrease observed in the major deep oceanic waters for the MPT time period. The phase relationship between aeolian terrigeneous signal increase and this δ13C decrease in our record, as well as in other paleorecords, supports the hypothesis of a global aridification amongst others processes to explain the deep-water masses isotopic signature changes during the MPT. In any case, the isotopic shifts imply major changes in the end-member δ18O and δ13C values of deep waters.

Monsoon-driven Saharan dust variability over the past 240,000 years

“…We interpret these differences as reflecting the impact of carbonate dissolution on dust percentages and thus age model–based MARs. As indicated by the carbon and oxygen isotopic ratios measured on MD03-2705 benthic foraminifera (δ 13 C ben and δ 18 O ben , respectively) ( 25 ), these three periods correspond to periods when δ 13 C ben is depleted and δ 18 O ben is enriched (Fig. 2).…”

“…( C ) Dust percentages (gray). ( D and E ) δ 18 O ben (green) and δ 13 C ben (orange) values from benthic foraminifera ( 25 ). ( F and G ) 230 Th-normalized CaCO 3 flux (blue) and total sediment flux (black).…”

“…2). These isotopic changes have been interpreted as representing deep ocean circulation changes that bring corrosive southern-sourced deep water to the site ( 25 ), leading to increased dissolution of the calcium carbonate deposited at the site. Reconstructions in the low- and mid-latitude North Atlantic indicate decreases in carbonate ion concentrations at similar depths (3.3 to 3.6 km) during MIS 2, 4, and 6 ( 26 , 27 ), consistent with increased carbonate dissolution at our core site at these times.…”

“…Sites ODP659 (18°04.63′N, 21°01.57′W, 3070 mbsl) and MD03-2705 (18°05.81′N, 21°09.19′W, 3085 mbsl) (Fig. 1) were drilled on a seamount off the Mauritanian coast, West Africa ( 3 , 25 ). This seamount culminates 300 m above the submarine ridge, connecting the Cape Verde Archipelago to the African margin ( 24 ).…”

“…CaCO 3 percentages for each sample were measured at Macalester College by coulometry, following methods in ( 7 ). Although eolian dust may contain detrital calcium carbonate, calcium carbonate contents at MD03-2705 are dominantly of marine biogenic origin, mainly foraminifera and coccoliths ( 25 ).…”