Thermohaline instability in the North Atlantic during meltwater events: Stable isotope and ice-rafted detritus records from 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 δ18O in TS diagrams in conjunction with equilibrium δc 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 δ13C by 0.7-1.1‰ during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic δ18O) at Site 609 and ventilation patterns (inferred from benthic δ13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic δ13C 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 δ13C 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 δ18O 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

Re-appraisal of the taxonomy of Acacia holosericea, including the description of a new species, A. Colei, and the reinstatement of A. Neurocarpa

The taxonomic status of Acacia holosericea A. Cunn. ex Don is re-appraised in the light of recent isozyme, chromosome and field studies, as well as from a critical examination of specimens at BM, BRI, K, DNA and PERTH. Accordingly, A. holosericea, as traditionally defined, is now regarded as comprising three distinct species, each corresponding to a different level of ploidy. Acacia holosericea A. Cunn. ex Don (tetraploid) is widespread, principally in tropical/subtropical areas of Western Australia, the Northern Territory and Queensland. Acacia neurocarpa A. Cunn. ex Hook, (diploid), occurs in tropical/subtropical Western Australia and the Northern Territory and until now has been regarded as conspecific with A. holosericea. Acacia colei Maslin and Thomson (hexaploid) is described as a new species which appears to have evolved as an allopolyploid hybrid between A. neurocarpa and A. cowleana (tetraploid). It is widespread and common in subtropical/arid Western Australia, the Northern Territory and Queensland. These four species are illustrated, mapped and their principal discriminating features given in tabular form and in a key. Acacia holosericea is neotypified to exclude A. neurocarpa, and A. neurocarpa is lectotypified to exclude A. dunnii (Maiden) Turrill. © 1992, CSIRO. All rights reserved

Variations in terrigenous dilution in western Mediterranean Sea pelagic sediments in response to climate change during the last glacial cycle

Hemipelagic intervals in four giant Calypso piston cores from the Balearic Abyssal Plain (western Mediterranean Sea) were studied in order to determine fluctuations in the supply of terrigenous sediments to the basin during the last 130,000 years and possible climatic modulation of terrigenous sediment and particle flux. Carbonate records from hemipelagic intervals in these cores display distinct ‘Atlantic’ type cycles, where glacial periods show, on average, 20% lower calcium carbonate contents than interglacial period equivalents. The decrease in calcium carbonate seen during glacials may be the result of increased dilution by terrigenous (aeolian and fluvial) particles derived from aeolian and/or fluvial sources. Higher glacial aeolian particle fluxes to the western Mediterranean Sea may have been caused by changes in the prevailing atmospheric conditions, resulting in drier and colder climatic conditions in the source areas and more frequent and intense outbreaks of dust transport. The colder and drier climatic conditions in combination with a lowered sea level would allow rivers to dump their sediment load closer to the shelf edge, promoting enhanced fluvial input into the basin. Besides significant calcium carbonate variations that correlate with glacial/interglacial cycles, several short-term abrupt changes in the calcium carbonate content are recorded, which appear coincident with enhanced magnetic susceptibility values, especially the events within Marine Isotope Stage (MIS) 3. The timing of these short-term events may be linked with cooling events in the North Atlantic, particularly Heinrich Events and cold events within MIS 5. These short-term trends may possibly be explained by increased terrigenous particle transport, due to drier and colder climate in the Mediterranean borderlands. Our results further show that about 90% of the sedimentary sequence of the Balearic Abyssal Plain consists of turbidites. Generally, these appear not to have caused significant erosion, and an almost complete continuous time stratigraphy is present in the intercalated hemipelagic intervals