ALBERT

Description: We present Plio-Pleistocene records of sediment color, %CaCO3, foraminifer fragmentation, benthic carbon isotopes (d13C) and radiogenic isotopes (Sr, Nd, Pb) of the terrigenous component from IODP Site U1313, a reoccupation of benchmark subtropical North Atlantic Ocean DSDP Site 607. We show that (inter)glacial cycles in sediment color and %CaCO3 pre-date major northern hemisphere glaciation and are unambiguously and consistently correlated to benthic oxygen isotopes back to 3.3 million years ago (Ma) and intermittently so probably back to the Miocene/Pliocene boundary. We show these lithological cycles to be driven by enhanced glacial fluxes of terrigenous material (eolian dust), not carbonate dissolution (the classic interpretation). Our radiogenic isotope data indicate a North American source for this dust (~3.3-2.4 Ma) in keeping with the interpreted source of terrestrial plant wax-derived biomarkers deposited at Site U1313. Yet our data indicate a mid latitude provenance regardless of (inter)glacial state, a finding that is inconsistent with the biomarker-inferred importance of glaciogenic mechanisms of dust production and transport. Moreover, we find that the relation between the biomarker and lithogenic components of dust accumulation is distinctly non-linear. Both records show a jump in glacial rates of accumulation from Marine Isotope Stage, MIS, G6 (2.72 Ma) onwards but the amplitude of this signal is about 3-8 times greater for biomarkers than for dust and particularly extreme during MIS 100 (2.52 Ma). We conclude that North America shifted abruptly to a distinctly more arid glacial regime from MIS G6, but major shifts in glacial North American vegetation biomes and regional wind fields (exacerbated by the growth of a large Laurentide Ice Sheet during MIS 100) likely explain amplification of this signal in the biomarker records. Our findings are consistent with wetter-than-modern reconstructions of North American continental climate under the warm high CO2 conditions of the Early Pliocene but contrast with most model predictions for the response of the hydrological cycle to anthropogenic warming over the coming 50 years (poleward expansion of the subtropical dry zones).

Description: Marine Isotope Stage (MIS) M2, 3.3 Ma, is an isolated cold stage punctuating the benthic oxygen isotope (δ¹⁸O) stratigraphy of the warm Piacenzian interval of the late Pliocene Epoch. The prominent (~0.65‰) δ¹⁸O increase that defines MIS M2 has prompted debate over the extent to which it signals an early prelude to the rhythmic extensive glaciations of the northern hemisphere that characterise the Quaternary and raised questions about the forcing mechanisms responsible. Recent work suggests that CO₂ storage in the deep Atlantic Ocean played an important role in these events but detailed reconstructions of deep ocean chemical stratification are needed to test this idea and competing hypotheses. Here we present new records of the Nd isotope composition of fish debris and δ¹³C and B/Ca ratios of benthic foraminifera from the northwest and southeast Atlantic Ocean. Our novel geochemical data show that, in contrast to major Quaternary glaciations such as MIS 2 (~21 ka) and MIS 100 (~2.52 Ma), the deep North Atlantic Ocean was weakly chemically stratified during MIS M2. We show that Southern Component Water incursion into the Atlantic Ocean was limited to the deep South Atlantic basin during MIS M2 and peaked well before (~10-15-kyr) the atmospheric CO₂ minimum. Our findings imply that the deep Atlantic Ocean was not the principle sink of CO₂ sequestered from the atmosphere during MIS M2, implicating a different CO₂ storage deep-water reservoir mechanism, presumably Southern Component Water incursion into the Pacific Ocean. Weak chemical stratification in the deep Atlantic Ocean during MIS M2 relative to MIS 100 and 2 suggests comparatively active Atlantic meridional overturning circulation. That suggestion is consistent with the warmth of the high latitude North Atlantic during MIS M2 - surface water temperatures cooled during M2 but only to Holocene values. Our findings may help to explain the paucity of evidence for extensive early glaciation of the northern hemisphere during M2 but leave open the possibility of ice sheet advance on Antarctica.