Sea surface temperature estimation for the last 70 ka from IODP Site 306-U1313

During the six Heinrich Events of the last 70 ka episodic calving from the circum-Atlantic ice sheets released large numbers of icebergs into the North Atlantic. These icebergs and associated melt-water flux are hypothesized to have led to a shutdown of Atlantic Meridional Overturning Circulation (AMOC) and severe cooling in large parts of the Northern Hemisphere. However, due to the limited availability of high-resolution records the magnitude sea surface temperature (SST) changes related to the impact of Heinrich Events on the mid-latitude North Atlantic is poorly constrained. Here we present a record of UK37'-based SSTs derived from sediments of Integrated Ocean Drilling Project (IODP) Site U1313, located at the southern end of the ice-rafted debris (IRD)-belt in the mid-latitude North Atlantic (41°N). We demonstrate that all six Heinrich Events are associated with a rapid warming of surface waters by 2 to 4°C in a few thousand years. The presence of IRD leaves no doubt about the simultaneous timing and correlation between rapid surface water warming and Heinrich Events. We argue that this warming in the mid-latitude North Atlantic is related to a northward expansion of the subtropical gyre during Heinrich Events. As a wide-range of studies demonstrated that in the central IRD-belt Heinrich Events are associated with low SSTs, these results thus identify an anti-phased (seesaw) pattern in SSTs during Heinrich Events between the mid-latitude (warm) and northern North Atlantic (cold). This highlights the complex response of surface water characteristics in the North Atlantic to Heinrich Events that is poorly reproduced by fresh water hosing experiments and challenges the widely accepted view that within the IRD-belt of the North Atlantic Heinrich Events coincide with periods of low SSTs.

Sea surface temperature estimation for MIS 16 of IODP Site 306-U1313

Hudson Strait (HS) Heinrich Events, ice-rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial-scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(-like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice-rafting debris (IRD), we detect the first HS Heinrich(-like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high-resolution alkenone-based SST record demonstrates that the first HS Heinrich(-like) Event did not coincide with low SSTs. Thus, the HS Heinrich(-like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid-Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.

Alkenone accumulation rates, stable carbon isotope record, and sea surface temperature reconstruction for IODP Site 306-U1313

Here we present orbitally-resolved records of terrestrial higher plant leaf wax input to the North Atlantic over the last 3.5 Ma, based on the accumulation of long-chain n-alkanes and n-alkanl-1-ols at IODP Site U1313. These lipids are a major component of dust, even in remote ocean areas, and have a predominantly aeolian origin in distal marine sediments. Our results demonstrate that around 2.7 million years ago (Ma), coinciding with the intensification of the Northern Hemisphere glaciation (NHG), the aeolian input of terrestrial material to the North Atlantic increased drastically. Since then, during every glacial the aeolian input of higher plant material was up to 30 times higher than during interglacials. The close correspondence between aeolian input to the North Atlantic and other dust records indicates a globally uniform response of dust sources to Quaternary climate variability, although the amplitude of variation differs among areas. We argue that the increased aeolian input at Site U1313 during glacials is predominantly related to the episodic appearance of continental ice sheets in North America and the associated strengthening of glaciogenic dust sources. Evolutional spectral analyses of the n-alkane records were therefore used to determine the dominant astronomical forcing in North American ice sheet advances. These results demonstrate that during the early Pleistocene North American ice sheet dynamics responded predominantly to variations in obliquity (41 ka), which argues against previous suggestions of precession-related variations in Northern Hemisphere ice sheets during the early Pleistocene.

Late Pliocene and Early Pleistocene dinoflagellate cysts and sea surface temperture reconstruction for several IODP and DSDP sites in the North Atlantic

In an attempt to document the palaeoecological affinities of individual extant and extinct dinoflagellate cysts, Late Pliocene and Early Pleistocene dinoflagellate cyst assemblages have been compared with geochemical data from the same samples. Mg/Ca ratios of Globigerina bulloides were measured to estimate the spring-summer sea-surface temperatures from four North Atlantic IODP/DSDP sites. Currently, our Pliocene-Pleistocene database contains 204 dinoflagellate cyst samples calibrated to geochemical data. This palaeo-database is compared with modern North Atlantic and global datasets. The focus lies in the quantitative relationship between Mg/Ca-based (i.e. spring-summer) sea-surface temperature (SSTMg/Ca) and dinoflagellate cyst distributions. In general, extant species are shown to have comparable spring-summer SST ranges in the past and today, demonstrating that our new approach is valid for inferring spring-summer SST ranges for extinct species. For example, Habibacysta tectata represents SSTMg/Ca values between 10° and 15°C when it exceeds 30% of the assemblage, and Invertocysta lacrymosa exceeds 15% when SSTMg/Ca values are between 18.6° and 23.5°C. However, comparing Pliocene and Pleistocene SSTMg/Ca values with present day summer values for the extant Impagidinium pallidum suggests a greater tolerance of higher temperatures in the past. This species occupies more than 5% of the assemblage at SSTMg/Ca values of 11.6-17.9°C in the Pliocene and Pleistocene, whereas present day summer SSTs are around -1.7 to 6.9°C. This observation questions the value of Impagidinium pallidum as reliable indicator of cold waters in older deposits, and may explain its bipolar distribution.

Biomarker and x-ray diffraction data from Site U1313 (MIS 16 - 9)

A reconstruction of Milankovitch to millennial-scale variability of sea-surface temperature (SST) and sea-surface productivity in the Pleistocene mid-latitude North Atlantic Ocean (MIS 16-9) and its relationship to ice sheet instability was carried out on sediments from IODP Site U1313. This reconstruction is based on alkenone and n-alkane concentrations, Uk37' index, total organic carbon (TOC) and carbonate contents, X-Ray diffraction (XRD) data, magnetic susceptibility, and accumulation rates. Increased input of ice-rafted debris (IRD) occurred during MIS 16, 12, and 10, characterized by high concentrations of dolomite, quartz, and feldspars and elevated accumulation rates of terrigenous matter. Minimum input values of terrigenous matter, on the other hand, were determined for MIS 13 and 11. Peak values of dolomite, coinciding with quartz, plagioclase, and kalifeldspar peaks and maxima in long-chain n-alkanes indicative for land plants, are interpreted as Heinrich-like Events related to sudden instability of the Laurentide Ice Sheet during early and late (deglacial) phases of the glacials. The coincidence of increased TOC values with elevated absolute concentrations of alkenones suggest increased glacial productivity, probably due to a more southern position of the Polar Front. Alkenone-based SST reached absolute maxima of about 19°C during MIS 11.3 and absolute minima of <10°C during MIS 12 and 10. Within MIS 11, prominent cooling events (MIS 11.22 and 11.24) occurred. The absolute SST minima recorded directly before and after the glacial maxima MIS 10.2 and 12.2, are related to Heinrich-like Event meltwater pulses, as supported by the coincidence of SST minima and maxima in C37:4 alkenones and dolomite. These sudden meltwater pulses - especially during Terminations IV and V - probably caused a collapse of phytoplankton productivity as indicated by the distinct drop in alkenone concentrations. Ice-sheet disintegration and subsequent surges and outbursts of icebergs and meltwater discharge may have been triggered by increased insolation in the Northern High Latitudes.