ALBERT

Description: The Denmark Strait Overflow (DSO) today compensates for the northward flowing Norwegian and Irminger branches of the North Atlantic Current that drive the Nordic heat pump. During the Last Glacial Maximum (LGM), ice sheets constricted the Denmark Strait aperture in addition to ice eustatic/isostatic effects which reduced its depth (today ∼630 m) by ∼130 m. These factors, combined with a reduced north-south density gradient of the water-masses, are expected to have restricted or even reversed the LGM DSO intensity. To better constrain these boundary conditions, we present a first reconstruction of the glacial DSO, using four new and four published epibenthic and planktic stable-isotope records from sites to the north and south of the Denmark Strait. The spatial and temporal distribution of epibenthic δ18O and δ13C maxima reveals a north-south density gradient at intermediate water depths from σ0∼28.7 to 28.4/28.1 and suggests that dense and highly ventilated water was convected in the Nordic Seas during the LGM. However, extremely high epibenthic δ13C values on top of the Mid-Atlantic Ridge document a further convection cell of Glacial North Atlantic Intermediate Water to the south of Iceland, which, however, was marked by much lower density (σ0∼28.1) The north-south gradient of water density possibly implied that the glacial DSO was directed to the south like today and fed Glacial North Atlantic Deep Water that has underthrusted the Glacial North Atlantic Intermediate Water in the Irminger Basin.

Description: The origin and potential impact of changes in the flow strength and salt discharge of Mediterranean Outflow Waters (MOW) on Atlantic Meridional Overturning Circulation have been long-standing objectives of (paleo-) oceanographers. Late Pliocene changes in the distal advection of MOW were reconstructed on orbital timescales for northeast Atlantic DSDP/ODP Site 548 off Brittany and Site 982 on Rockall Plateau. The proxy records were compared to Western Mediterranean deep-water records of Alboran Sea Site 978 and now supplemented by a MOW record from Site U1389 directly west off Gibraltar. At sites 548 and 982, MOW temperatures and salinities form a prominent rise by 2°–4°C and ~3 psu from ~3.43–3.3 Ma, induced by a preceding and coeval rise in sea surface and deep-water salinity and increased summer aridity in the Western Mediterranean Sea. We suggest that these changes triggering an increased MOW flow were ultimately induced by a persistent 4°C cooling of Indonesian Throughflow waters (Karas et al., 2011). First postulated by Cane and Molnar (2001), the temperature drop resulted from the northward drift of Australia that crossed a threshold value near 3.6–3.3 Ma and led to a large-scale cooling of the eastern subtropical Indian Ocean and in turn, to a reduction of African monsoon rains. Vice versa, we show that the distinct rise in Mediterranean salt export after ~3.4 Ma induced an expected unique long-term rise in the formation of Upper North Atlantic Deep Water, that followed with a phase lag of ~100 ky, finally inducing glacial MIS M2.

Description: Five Ocean Drilling Program sites (657-661), which form a north-south transect off the western periphery of the Sahara, were selected. Nearshore mean flux of opal off Cap Blanc (21°N) showed an abrupt increase about 3 Ma that appears to reflect the main onset of coastal upwelling fertility and enhanced trade winds. At the same time, the input of river-borne clay strongly decreased, suggesting a dry up of the central Saharan rivers. Later, marked short-lived spikes of clay and opal may indicate ongoing ephemeral pulses of fluvial runoff linked to peak interglacial stages. Aridification of the south Sahara and Sahel increased in several steps: at 4.6, 4.3 and especially at 4.0, 3.6 and 2.1 Ma, and again, at 0.8 Ma. The late Miocene and earliest Pliocene were humid. Central and north Sahara climate appears to be linked to the glaciation history of the Northern Hemisphere. Spatial distribution of quartz accumulation suggests that the dust outbreaks linked to the Intertropical Convergence Zone during summer did not shift in latitude back to 4.0 Ma, at least. Short-term variations of dust output over the last 0.5 my followed orbital scale pulses with a strong precessional signal, showing a link of Sahelian humidity changes to the variation of sea-surface temperature and evaporation in the tropical Atlantic.

Description: Largely continuous millennial-scale records of benthic delta O-18, Mg/Ca-based temperature, and salinity variations in bottom waters were obtained from Deep Sea Drilling Project (DSDP) Site 548 (East Atlantic continental margin near Ireland, 1250 m water depth) for the period 3.7-3.0 Ma ago. High epsilon(Nd) values of -10.7 to -9 show that this site monitored changes in Mediterranean Outflow Water (MOW) throughout the mid-Pliocene. Bottom water variability at Ocean Drilling Progam (ODP) Site 978 (Alboran Sea, 1930 m water depth) provides a complementary record of MOW composition near its West Mediterranean source. Both sites show a singular and persistent rise in bottom water salinities by 0.7-1.4 psu, and in densities by similar to 1 kg m(-3) from 3.5 to 3.3 Ma ago, which is matched by an similar to 3 degrees C increase in bottom water temperature at Site 548. This event suggests the onset of strongly enhanced deep-water convection in the Mediterranean Sea and a related increase in MOW flow as a result of major aridification in the Mediterranean source region. In harmony with model suggestions, the enhanced MOW flow has possibly intensified Upper North Atlantic Deep Water formation.

Description: Oceanic plankton (export) productivity contributes to the control of glacial-to-interglacial changes in atmospheric CO2 concentration. The extent of this contribution may be deciphered from global reconstructions of palaeoproductivity. We quantitatively estimate palaeoproductivity over the last 350 000 years in the eastern equatorial Atlantic, using equations based on foraminiferal assemblages and marine organic carbon accumulation rates; and make qualitative estimates using diatom and radiolarian accumulation rates. These proxydata are calibrated to data on modern primary production.

Description: Eolian-sand turbidites form a clearly distinguishable "eolomarine" sediment facies. They are almost devoid of gradation, fine fraction, and mica, and have comparatively coarse sand medians. The sands consist predominantly of quartz grains, a significant portion of which show yellowish-red stains and frosted surfaces, both characteristic of desert sands in subtropical latitudes. The turbidite beds frequently reach thicknesses of 100 to 〉600 cm. A persistent fraction of calcareous shallow-water particles indicates a shoreline derivation. Eolian-sand turbidites occur off passive continental margins where active desert dunes have migrated seaward by dominantly offshore winds during (glacial) periods of low sea level. Accordingly, they indicate and date past environmental zones of minimum precipitation that migrate rapidly over the land and leave few datable traces. For example, during Pleistocene Weichselian time turbidites in the Gulf of Guinea resulted from a northwestward shift of dunes of the Kalahari desert across the Congo River. Similarly, an Early Miocene precursor of the Sahara is suggested for the region of the Spanish Sahara which then lay at 15-16 degrees N. This appears to be a Miocene analogue of the Pleistocene Weichselian desert. A model for the initiation of eolian-sand turbidity currents can start with the Recent situation of high sea level in northwest Africa where desert-dune supplied sand is trapped in a sand wedge at the shoreline, slowly prograding over the shelf platform. However, during a glacial low-sea-level stage, this wedge was located at, or below, the shelfbreak, on gradients of which 37% exceed 10 degrees at the ubiquitous slope-incision heads. A series of structural, petrographical and environmental features then generated repeated failures and slumps of gigantic quantities of sand that became turbidity currents. As they moved downslope, they may have cut or kept open numerous small slope incisions. On the continental rise they formed highly porous deposits of up to hundreds of cubic kilometers with a high potential storage capacity for hydrocarbons.

Description: 50 m of Middle Eocene pure radiolarian ooze were drilled at ODP Site 660 in the equatorial East Atlantic, 80 km northeast of the Kane Gap. The oozes comprise a 10 m high and 2 km broad mound of seismic reverberations, covered by manganese-rich sediment, and contain trace amounts of sponge spicules and diatoms, negligible organic carbon (0.15%), clay, and variable amounts of pyrite. The yellow to pale brown silty sediments are relatively coarse-grained (30–45% coarser than 6 μm), little bioturbated, and commonly massive or laminated on a cm-scale. The unlithified radiolarian ooze may indicate an interval of high oceanic productivity, probably linked to a palaeoposition of Site 660 close to the equatorial upwelling belt during Middle Eocene time. The absence of organic matter, however, and both the laminated bedding and the mound-like structure of the deposit on the lower slope of a continental rise indicate deposition by relatively intense contour currents of oxygen-rich deep water, which passed through the Kane Gap, winnowed the fine clay fraction, and prevented the preservation of organic carbon. The ooze may be either a contourite-lag deposit, or a contourite accumulation of displaced radiolarians, originating south of the Kane Gap and being deposited in its northern lee, thus documenting the passage of a strong cross-equatorial bottom-water current formed near Antarctica. These Eocene contourites may be an analogue for ancient radiolarites in the Tethyan Ocean.

Description: High-resolution planktonic and epibenthic stable isotope records from Ocean Drilling Program site 658 off northwest Africa provide a basis for a detailed study of glacial terminations I-VI during the last 650,000 years. The duration of the terminations was about one half to one quarter of an orbital precession cycle (5800–10,700 years), when its amplitude was high. At low amplitudes, the terminations lasted longer than half an obliquity cycle (29,000 years). Marked climatic rebounds similar to the Younger Dryas, each with a duration of 1000–2500 years, subdivided all six terminations into distinct steps A, B, and C. Important parts of the deglacial steps were as brief as 700–1000 years. The speed of climatic change suggests that special associations existed between orbital forcing and inherent instability of the ice sheets. In harmony with published models, the more rapid pulses of glacial meltwater incursions to the northern North Atlantic led to one or more brief short-term shut-downs of North Atlantic Deep Water formation. This process is reflected by pronounced benthic δ13C minima that precisely coincide, in most cases, with the end of the δ18O deglaciation steps and immediately terminate with succeeding Younger Dryas-style cooling events. Thus we conclude that the rebounds resulted from a short-term antecedent estuarine circulation regime in the North Atlantic, except for the Younger Dryas itself, which succeeded the Alleröd with its well ventilated Atlantic deepwater circulation and hence continues to be an enigma.

Description: Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ13C record of CO2, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene. With the onset of Termination I about 15,000 years ago, the new (export) production of low- and mid-latitude upwelling cells started to decline by more than 2-4 Gt carbon/year. This reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric CO2 as recorded in ice cores and, with a slight delay of more than 1000 years, to a large-scale gradual CO2 depletion of the deep ocean by about 650 Gt C. This estimate is based on an average increase in benthic δ13C by 0.4–0.5‰. The decrease in new production also matches a clear 13C depletion of organic matter, possibly recording an end of extreme nutrient utilization in upwelling cells. As shown by Sarnthein et al., [1987], the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades, which in turn was linked via a shrinking extent of sea ice to a massive increase in high-latitude insolation, i.e., to orbital forcing as primary cause.