Lucia de Abreu

Millennial-scale oceanic climate variability off the Western Iberian margin during the last two glacial periods

High-resolution palaeoclimate records recovered from the Iberian margin in core MD95-2040 exhibit large fluctuations in oceanographic conditions over the last 190 ka. Large-scale cooling of the surface ocean is indicated by the presence of the polar planktonic foraminifer Neogloboquadrina pachyderma (sinistral), and in some instances the occurrence of ice-rafted debris (IRD). Ice-rafting episodes were prevalent in both of the last two glacials with greater intensity in Stages 2 through 4, than in Stage 6. The six youngest Heinrich events are well defined during the last glacial but detrital carbonate is absent from Heinrich layers HL6, HL5 and HL3. Dansgaard–Oeschger stadial-equivalent sub-millennial IRD deposition events have been detected, in particular during Stage 3, allowing a good match with the cooling displayed in the Greenland ice core (GISP2). Sea-surface temperature off Portugal in Stage 6 was in general warmer than during the last glacial, pointing towards a weaker southward influence of polar water masses. Ice rafting occurred mainly in mid-MIS (Marine Isotope Stage) 6 (between 173 and 153 kyr) as a group of poorly differentiated, short-duration quasi-continuous events, mainly marked by the high abundance of sinistral N. pachyderma. Differences exist in IRD composition relative to the last glacial, with a reduced Canadian-derived detrital carbonate component, combined with an important contribution of volcanic particles. The lower magnitude and higher frequency of these events suggest that the higher temperatures would have induced iceberg waning closer to the source areas.

Surface and deep water response to rapid climate changes at the Western Iberian Margin

Rapid climate changes at the onset of the last deglaciation and during Heinrich Event H4 were studied in detail at IMAGES cores MD95-2039 and MD95-2040 from the Western Iberian margin. A major reorganisation of surface water hydrography, benthic foraminiferal community structure, and deepwater isotopic composition commenced already 540 years before the Last Isotopic Maximum (LIM) at 17.43 cal. ka and within 670 years affected all environments. Changes were initiated by meltwater spill in the Nordic Seas and northern North Atlantic that commenced 100 years before concomitant changes were felt off western Iberia. Benthic foraminiferal associations record the drawdown of deepwater oxygenation during meltwater and subsequent Heinrich Events H1 and H4 with a bloom of dysoxic species. At a water depth of 3380 m, benthic oxygen isotopes depict the influence of brines from sea ice formation during ice-rafting pulses and meltwater spill. The brines conceivably were a source of ventilation and provided oxygen to the deeper water masses. Some if not most of the lower deep water came from the South Atlantic. Benthic foraminiferal assemblages display a multi-centennial, approximately 300-year periodicity of oxygen supply at 2470-m water depth. This pattern suggests a probable influence of atmospheric oscillations on the thermohaline convection with frequencies similar to Holocene climate variations. For Heinrich Events H1 and H4, response times of surface water properties off western Iberia to meltwater injection to the Nordic Seas were extremely short, in the range of a few decades only. The ensuing reduction of deepwater ventilation commenced within 500–600 years after the first onset of meltwater spill. These fast temporal responses lend credence to numerical simulations that indicate ocean–climate responses on similar and even faster time scales.

Ocean climate variability in the eastern North Atlantic during interglacial marine isotope stage 11: A partial analogue to the Holocene?

Similar orbital geometry and greenhouse gas concentrations during marine isotope stage 11 ( MIS 11) and the Holocene make stage 11 perhaps the best geological analogue period for the natural development of the present interglacial climate. Results of a detailed study of core MD01-2443 from the Iberian margin suggest that sea surface conditions during stage 11 were not significantly different from those observed during the elapsed portion of the Holocene. Peak interglacial conditions during stage 11 lasted nearly 18 kyr, indicating a Holocene unperturbed by human activity might last an additional 6-7 kyr. A comparison of sea surface temperatures ( SST) derived from planktonic foraminifera for all interglacial intervals of the last million years reveals that warm temperatures during peak interglacials MIS 1, 5e, and 11 were higher on the Iberian margin than during substage 7e and most of 9e. The SST results are supported by heavier delta(18)O values, particularly during 7e, indicating colder SSTs and a larger residual ice volume. Benthic delta(13)C results provide evidence of a strong influence of North Atlantic Deep Water at greater depths than present during MIS 11. The progressive ocean climate deterioration into the following glaciation is associated with an increase in local upwelling intensity, interspersed by periodic cold episodes due to ice- rafting events occurring in the North Atlantic.

A Review of Abrupt Climate Change Events in the Northeastern Atlantic Ocean (Iberian Margin): Latitudinal, Longitudinal, and Vertical Gradients

Abrupt Climate C Geophysical Mon Copyright 2011 b 10.1029/2010GM The Iberian margin is a key location to study abrupt glacial climate change, and regional variability is studied combining published and new records. Looking at the trend from marine isotope stage (MIS) 10 to 2, the planktic foraminifer data, conforming to Martrat et al. [2007], show that abrupt events, especially Heinrich events, became more frequent and their impacts stronger during the last glacial cycle. However, there were two older periods with strong impacts on the Atlantic meridional overturning circulation: the Heinrich-type event associated with termination IV and the one occurring during MIS 8 (269 to 265 ka). During Heinrich stadials, the Polar Front reached the northern Iberian margin (approximately 41°N), while the Arctic Front was located in the vicinity of 39°N. During all glacial periods, 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 the fronts, sea surface temperature (SST) increased southward by about 1°C per 1° latitude leading to steep SST gradients. Glacial hydrographic conditions were similar during MIS 2 and 4 but much different during MIS 6. MIS 6 was a warmer glacial with subtropical waters reaching as far north as 40.6°N. In the vertical structure, Greenland-type oscillations were recorded down to 2465 m during Heinrich stadials, i.e., deeper than in the western basin, due to the admixing of Mediterranean Outflow Water. It is evident that latitudinal, longitudinal, and