Age determination and Heinrich Events in sediment cores SU90-08 and KET80-03

The Mediterranean Sea hydrology at the time of the Heinrich formation in the North Atlantic Ocean was analyzed by comparing sea surface temperatures (SSTs) and oxygen isotope composition of seawater (dw) changes during the past 75 kyr in two marine cores. These were compared to the palynological variations derived in the Mediterranean Sea core. During the last glacial the two oceanic SST records show similar and synchronous patterns, with several long-term cooling periods, ending by abrupt SST increases. At the time of the Heinrich events, cold SSTs and low salinity prevailed in the Mediterranean Sea. The freshwater budget was similar to the modern one, permitting the presence of a mixed forest on the Mediterranean borderlands. The post-Heinrich periods are marked by a freshwater budget decrease, limiting oak and fir tree growth in the Mediterranean region. Increase of precipitation or reduction of evaporation is observed before the Heinrich episode, and is associated with a well-developed mixed Mediterranean forest.

North Atlantic IODP Site 303-U1308 Middle Pleistocene Heinrich Events record

Heinrich events are well documented for the last glaciation, but little is known about their occurrence in older glacial periods of the Pleistocene. Here we report scanning XRF and bulk carbonate d18O results from Integrated Ocean Drilling Program Site U1308 (reoccupation of Deep Sea Drilling Project Site 609) that are used to develop proxy records of ice-rafted detritus (IRD) for the last ~1.4 Ma. Ca/Sr is used as an indicator of IRD layers that are rich in detrital carbonate (i.e., Heinrich layers), whereas Si/Sr reflects layers that are poor in biogenic carbonate and relatively rich in detrital silicate minerals. A pronounced change occurred in the composition and frequency of IRD at ~640 ka during marine isotope stage (MIS) 16, coinciding with the end of the middle Pleistocene transition. At this time, "Hudson Strait" Heinrich layers suddenly appeared in the sedimentary record of Site U1308, and the dominant period of the Si/Sr proxy shifted from 41 ka prior to 640 ka to 100 ka afterward. The onset of Heinrich layers during MIS 16 represents either the initiation of surging of the Laurentide Ice Sheet (LIS) off Hudson Strait or the first time icebergs produced by this process survived the transport to Site U1308. We speculate that ice volume (i.e., thickness) and duration surpassed a critical threshold during MIS 16 and activated the dynamical processes responsible for LIS instability in the region of Hudson Strait. We also observe a strong coupling between IRD proxies and benthic d13C variation at Site U1308 throughout the Pleistocene, supporting a link between iceberg discharge and weakening of thermohaline circulation in the North Atlantic.

Aprubt climate change events in the Northeastern Atlantic Ocean

The western Iberian margin has been one of the key locations to study abrupt glacial climate change and associated interhemispheric linkages. The regional variability in the response to those events is being studied by combining a multitude of published and new records. Looking at the trend from Marine Isotope Stage (MIS) 10 to 2, the planktic foraminifer data, conform with the alkenone record of Martrat et al. [2007], shows that abrupt climate change events, especially the Heinrich events, became more frequent and their impacts in general stronger during the last glacial cycle. However, there were two older periods with strong impacts on the Atlantic meridional overturning circulation (AMOC): the Heinrich-type event associated with Termination (T) IV and the one occurring during MIS 8 (269 to 265 ka). During the Heinrich stadials of the last glacial cycle, the polar front reached the northern Iberian margin (ca. 41°N), while the arctic front was located in the vicinity of 39°N. During all the glacial periods studied, there existed a boundary at the latter latitude, either the arctic front during extreme cold events or the subarctic front during less strong coolings or warmer glacials. Along with these fronts sea surface temperatures (SST) increased southward by about 1°C per one degree of latitude leading to steep temperature gradients in the eastern North Atlantic and pointing to a close vicinity between subpolar and subtropical waters. The southern Iberian margin was always bathed by subtropical water masses - surface and/ or subsurface ones -, but there were periods when these waters also penetrated northward to 40.6°N. Glacial hydrographic conditions were similar during MIS 2 and 4, but much different during MIS 6. MIS 6 was a warmer glacial with the polar front being located further to the north allowing the subtropical surface and subsurface waters to reach at minimum as far north as 40.6°N and resulting in relative stable conditions on the southern margin. In the vertical structure, the Greenland-type climate oscillations during the last glacial cycle were recorded down to 2465 m during the Heinrich stadials, i.e. slightly deeper than in the western basin. This deeper boundary is related to the admixing of Mediterranean Outflow Water, which also explains the better ventilation of the intermediate-depth water column on the Iberian margin. This compilation revealed that latitudinal, longitudinal and vertical gradients existed in the waters along the Iberian margin, i.e. in a relative restricted area, but sufficient paleo-data exists now to validate regional climate models for abrupt climate change events in the northeastern North Atlantic Ocean.

(Appendix A) Oxygen and carbon isotope results of detrital carbonate grains of Heinrich layers from HU90-013-028 and IODP Sites 303-U1302/U1303/U1308 in the North Atlantic Ocean

We report oxygen and carbon isotope results of detrital carbonate grains from Heinrich layers at three sites in the North Atlantic located along a transect from the Labrador Sea to the eastern North Atlantic. Oxygen isotopic values of individual detrital carbonate grains from six Heinrich layers at all sites average - 5.6 ppm ± 1.5 ppm (1sigma; n = 166), reflecting values of dolomitic limestone derived from source areas in northeastern Canada. The d18O of bulk carbonate at Integrated Ocean Drilling Program (IODP) Site U1308 (re-occupation of Deep Sea Drilling Project (DSDP) Site 609) in the eastern North Atlantic records the proportion of detrital to biogenic carbonate and d18O decreases to - 5 ppm during Heinrich (H) events 1, 2, 4 and 5 relative to a background value of ~ 1 to 2 ppm for biogenic carbonate. Bulk d18O also decreases during H3 and H6 but only attains values of - 1ppm, indicating either a greater proportion of biogenic-to-detrital carbonate or a different source. Because the d18O of detrital carbonate is ~ 9 ppm lower than foraminifer carbonate, any fine-grained detrital carbonate not removed from the inner test chambers will lower foraminifer d18O. We conclude bulk carbonate d18O is a sensitive proxy for detrital carbonate and may be useful for identifying Heinrich layers in cores within and near the margins of the North Atlantic ice-rafted detritus (IRD) belt.

Geochemistry and sea-surface temperature reconstruction for sediment cores of the Iberian Margin

Biogenic records of the marine palaeoproductivity (carbonates, organic carbon, and C37 alkenones) and the molecular stratigraphy of past sea surface temperatures (SSTs; UK'37) were studied at high resolution in two cores of the Iberian Margin. The comparison of these records indicates that the oceanographic conditions switched abruptly during the past 160 kyr between three kinds of regimes. A first regime with high (17-22°C) SST and low productivity typifies the interglacial periods, marine isotopic stages (MIS) 5 and 1. Several periods during MIS 6, 2, and the terminations II and I are characterised by about 4-5°C colder SST and a higher organic matter accumulation, both of which define the second regime. This anticorrelation between SST and marine productivity suggests that these variations are related to the intensity of the coastal upwelling. By contrast with this upwelling behaviour, extremely low biological productivity and very cold SST (6-12°C) occurred during short phases of glacial MIS 6, 4, and 2, and as abrupt events (~1 kyr or less) during MIS 3. The three oceanographic regimes are consistent with micropalaeontological changes in the same cores based on foraminifera and diatoms. The general trend of these hydrologic changes follows the long-term glacial/interglacial cycle, but the millennium scale variability is clearly related to Heinrich events and Dansgaard-Oeschger cycles. Strengthening of the upwelling corresponds probably to an intensification of the subtropical atmospheric circulation over the North Atlantic which was influenced by the presence of continental ice sheets. However, extreme glacial conditions due to massive discharges of icebergs interrupted the upwelling. Interestingly, both terminations II and I coincided with strong but transient intensification of the upwelling.

Mineralogy and age determination of sediment core D11957P

The Tore Seamount is a circular, volcano-like feature 100 km in diameter with its summit at 2200 m water depth and a small, 5000 m deep basin in its interior. It is situated approximately 300 km west of Lisbon and is surrounded by deep abyssal plains. This site with a standard pelagic stratigraphy is the southernmost point where the so-called Heinrich events have so far been recorded. A succession of alternating interglacial/glacial periods reveals a stratigraphic record back to the beginning of isotopic stage 7 (225 kyr). Climatic changes are identifiable by coherent variations in colour, carbonate content and distribution of ice-rafted detritus in the carbonate-free fraction. Inputs of ice-rafted quartz are well defined. Characteristics in common with other sites showing Heinrich layers include a high terrigenous to biogenic ratio, a dramatic decrease in the accumulation rate of foraminifera shells, an increase in dolomite abundance and the occurrence of polar foraminiferal species indicating southwards penetration of cold waters which lead us to consider a wider southeastern extent of the North Atlantic ice-rafted detritus belt than hitherto. If the presently accepted position of the Polar Front is maintained, icebergs must have been swept southwards from the southern boundary of the pack ice in a current merging into the ancestral Canary Current, bringing ice-rafted material to the Tore Seamount. The coincidence of reddish-feldspar, probably derived from the northern Appalachian Triassic red facies, with the transparent quartz suggests at least a partial Labrador source for all the Heinrich layers here, including HL 3. In comparison to other sites in the entire North Atlantic, two exceptions stand out: the absence of HL 5 and the low detritus to biogenics ratio for HL 3. The simultaneous occurrence of these two types of ice-rafted minerals is a new piece in the puzzle of the origin of Heinrich layers.

Radiocarbon dates of sediments from ODP Site 161-976

Multiproxy paleoenvironmental records (pollen and planktonic isotope) from Ocean Drilling Program Site 976 (Alboran Sea) document rapid ocean and climate variations during the last glacial that follow the Dansgaard-Oeschger climate oscillations seen in the Greenland ice core records, thus suggesting a close link of the Mediterranean climate swings with North Atlantic climates. Continental conditions rapidly oscillated through cold-arid and warm-wet conditions in the course of stadial-interstadial climate jumps. At the time of Heinrich events, i.e., maximum meltwater flux to the North Atlantic, western Mediterranean marine microflora and microfauna show rapid cooling correlated with increasing continental dryness. Enhanced aridity conceivably points to prolonged wintertime stability of atmospheric high-pressure systems over the southwestern Mediterranean in conjunction with cooling of the North Atlantic.

High-resolution marine record of sediment core MD99-2343 in the Balearic Sea

The IMAGES core MD99-2343, recovered from a sediment drift north of the island of Minorca, in the north-western Mediterranean Sea, holds a high-resolution sequence that is perfectly suited to study the oscillations of the overturning system of the Western Mediterranean Deep Water (WMDW). Detailed analysis of grain-size and bulk geochemical composition reveals the sensitivity of this region to climate changes at both orbital and centennial-millennial temporal scales during the last 50 kyr. The dominant orbital pattern in the K/Al record indicates that sediment supply to the basin was controlled by the insolation evolution at 40°N, which forced changes in the fluvial regime, with more efficient sediment transport during insolation maxima. This orbital control also modulated the long-term pattern of the WMDW intensity as illustrated by the silt/clay ratio. However, deep convection was particularly sensitive to climatic changes at shorter time-scales, i.e. to centennial-millennial glacial and Holocene oscillations that are well documented by all the paleocurrent intensity proxies (Si/Al, Ti/Al and silt/clay ratios). Benthic isotopic records (d13C and d18O) show a Dansgaard-Oeschger (D-O) pattern of variability of WMDW properties, which can be associated with changing intensities of the deep currents system. The most prominent reduction on the WMDW overturning was caused by the post-glacial sea level rise. Three main scenarios of WMDW overturning are revealed: a strong mode during D-O Stadials, a weak mode during D-O Interstadials and an intermediate mode during cooling transitions. In addition, D-O Stadials associated with Heinrich events (HEs) have a very distinct signature as the strong mode of circulation, typical for the other D-O Stadials, was never reached during HE due to the surface freshening induced by the inflowing polar waters. Consequently, the WMDW overturning system oscillated around the intermediate mode of circulation during HE. Though surface conditions were more stable during the Holocene, the WMDW overturning cell still reacted synchronously to short-lived events, as shown by increments in the planktonic d18O record, triggering quick reinforcements of the deep water circulation. Overall, these results highlight the sensitivity of the WMDW to rapid climate change which in the recent past were likely induced by oceanographic and atmospheric reorganizations in the North Atlantic region.

Physical properties of sediment cores from the Labrador Sea

Sediment core logs from six sediment cores in the Labrador Sea show millennial-scale climate variability during the last glacial by recording all Heinrich events and several major Dansgaard-Oeschger cycles. The same millennial-scale climate change is documented for surface-water d18O records of Neogloboquadrina pachyderma (left coiled); hence the surface-water d18O record can be derived from sediment core logging by means of multiple linear regression, providing a paleoclimate proxy record at very high temporal resolution (70 yrs). For the Labrador Sea, sediment core logs contain important information about deep-water current velocities and also reflect the variable input of IRD from different sources as inferred from grain-size analysis, benthic d18O, the relation of density and p-wave velocity, and magnetic susceptibility. For the last glacial, faster deep-water currents which correspond to highs in sediment physical properties, occurred during iceberg discharge and lasted for a several centuries to a few millennia. Those enhanced currents might have contributed to increased production of intermediate waters during times of reduced production of North Atlantic Deep Water. Hudson Strait might have acted as a major supplier of detrital carbonate only during lowered sea level (greater ice extent). During coldest atmospheric temperatures over Greenland, deep-water currents increased during iceberg discharge in the Labrador Sea, then surface water freshened shortly after, while the abrupt atmospheric temperature rise happened after a larger time lag of >=1 kyr. The correlation implies a strong link and common forcing for atmosphere, sea surface, and deep water during the last glacial at millennial time scales but decoupling at orbital time scales.

Age determination of sediment core SO75-26KL, Portuguese margin

A benthic isotope record has been measured for core SO75-26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic delta18O in TS diagrams in conjunction with equilibrium deltac fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (< 15% compared to 30% today). The benthic isotope record along core SO75-26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C-accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice-rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic delta13C by 0.7-1.1 per mil during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic delta18O) at Site 609 and ventilation patterns (inferred from benthic delta13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic delta13C starts to decrease 1.5-2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5-3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic delta13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin-up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core delta18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record.