Stable isotope record and aklenonen index of MIS 11 sediments

The interglacial known as Marine Isotope Stage 11 has been proposed to be analogous to the Holocene, owing to similarities in the amplitudes of orbital forcing. It has been difficult to compare the periods, however, because of the long duration of Stage 11 and a lack of detailed knowledge of any extreme climate events that may have occurred. Here we use the distinctive phasing between seasurface temperatures and the oxygen-isotope records of benthic foraminifera in the southeast Atlantic Ocean to stratigraphically align the Holocene interglacial with the first half of the Marine Isotope Stage 11 interglacial optimum. This alignment suggests that the second half of Marine Isotope Stage 11 should not be used as a reference for 'pre-anthropogenic' greenhouse-gas emissions. By compiling benthic carbon-isotope records from sites in the Atlantic Ocean on a single timescale, we also find that meridional overturning circulation strengthened about 415,000 years ago, at a time of high orbital obliquity. We propose that this mechanism transported heat to the high northern latitudes, inhibiting significant ice-sheet build-up and prolonging interglacial conditions. We suggest that this mechanism may have also prolonged other interglacial periods throughout the past 800,000 years.

(Table 1) Alkenone concentration, UK'37 index and derived sea surface temperatures for ODP Site 167-1020

Uk'37 sea-surface temperature (SST) estimates obtained at ~2.5-k.y. resolution from Ocean Drilling Program Site 1020 show glacial-interglacial cyclicity with an amplitude of 7°-10°C over the last 780 k.y. This record shows a similar pattern of variability to another alkenone-based SST record obtained previously from the Santa Barbara Basin. Both records show that oxygen isotope Stage (OIS) 5.5 was warmer by ~3°C relative to the present and that glacial Uk'37 temperatures warm in advance of deglaciation, as inferred from benthic d18O records. The alkenone-based SST record at Site 1020 is longer than previously published work along the California margin. We show that warmer than present interglacial stages have occurred frequently during the last 800 k.y. Alkenone concentrations, a proxy for coccolithophorid productivity, indicate that sedimentary marine organic carbon content has also varied significantly over this interval, with higher contents during interglacial periods. A baseline shift to warmer SST and greater alkenone content occurs before OIS 13. We compare our results with those from previous multiproxy studies in this region and conclude that SST has increased by ~5°C since the last glacial period (21 ka). Our data show that maximum alkenone SSTs occur simultaneously with minimum ice volume at Site 1020, which is consistent with data from farther south along the margin. The presence of sea ice in the glacial northeast Pacific, the extent of which is inferred from locations of ice-rafted debris, provides further support for our notion of cold surface water within the northern California Current system, averaging 7°-8°C cooler during peak glacial conditions. The cooling of surface water during glacial stages most likely did not result from enhanced upwelling because alkenone concentrations and terrestrial redwood pollen assemblages are consistently lower during glacial periods.