Search

The southern westerly wind belt (SWW) interacts with the Antarctic Circumpolar Current and strongly impacts the Southern Ocean carbon budget, and Antarctic ice-sheet dynamics across glacial-interglacial cycles. We investigated precipitation-driven sediment input changes to the Southeast Pacific off the southern margin of the Atacama Desert over the past one million years, revealing strong precession (19/23-ka) cycles. Our simulations with 2 ocean-atmosphere general circulation models suggest that observed cyclic rainfall changes are linked to meridional shifts in water vapor transport from the tropical Pacific toward the southern Atacama Desert. These changes reflect a precessional modulation of the split in the austral winter South Pacific jet stream. For precession maxima, we infer significantly enhanced rainfall in the southern Atacama Desert due to a stronger South Pacific split jet with enhanced subtropical/subpolar jets, and a weaker midlatitude jet. Conversely, we derive dry conditions in northern Chile related to reduced subtropical/subpolar jets and an enhanced midlatitude jet for precession minima. The presence of precessional cycles in the Pacific SWW, and lack thereof in other basins, indicate that orbital-scale changes of the SWW were not zonally homogeneous across the Southern Hemisphere, in contrast to the hemispherewide shifts of the SWW suggested for glacial terminations. The strengthening of the jet is unique to the South Pacific realm and might have affected winter-controlled changes in the mixed layer depth, the formation of intermediate water, and the buildup of sea-ice around Antarctica, with implications for the global overturning circulation and the oceanic storage of atmospheric CO2.

Present-day sediment influx to the northern Red Sea is dominated by aeolian dust because of its position between the large deserts of northern Africa, the Arabian Peninsula and the Levant, and the absence of discharge from perennial rivers. However, sediment cores retrieved from the northern Red Sea reveal strong temporal variability of dust influx to the basin on glacial-interglacial timescales and several shorter-term strong episodes of fluvial input. We report new palaeoclimate and sediment provenance records for the last ca. 220 kyr from marine sediment core KL23, retrieved from the northern part of this basin. Our data suggest that the Nile delta became a major dust source during glacial conditions, in response to the glacioeustatic sea-level fall and associated subaerial exposure of volcanic-rich debris originally transported down the river Nile from the Ethiopian Highlands. Windblown dust from this delta source is characterized by high smectite concentrations and Ti contents. It is transported to the northern Red Sea on prevailing NNW winds. Our data also suggest a contribution of kaolinite-rich windblown dust from Egypt, Sinai and the Levant to KL23 with the same winds. The activity of this source is hydrologically controlled, with minima in kaolinite concentrations documenting phases of increased humidity, probably due to enhanced Mediterranean cyclogenesis and a southward expansion of the Mediterranean winter rains. Short-term reactivations of wadi systems during fluvial episodes are identified by fine grain sizes, high chlorite concentrations and high εNd. These episodes correlate with phases of reduced aeolian influx to the northern Red Sea and coincide with African Humid Periods, both in timing and relative intensity. This result implies that the Mediterranean climate system and the African monsoon are closely coupled. [ABSTRACT FROM AUTHOR]

Calcium carbonate (CaCO3) dissolution at the Southern Ocean seafloor has hypothetically contributed to lowering the atmospheric carbon dioxide concentration by increasing ocean alkalinity during glacial periods. We present new CaCO3 burial and dissolution records from two sediment cores obtained off the Chilean margin in the subantarctic SE Pacific and covering the past 140 kyr since Marine Isotope Stage (MIS) 6. These records include CaCO3 contents and mass accumulation rates, and microfossil-based analysis results, including fragmentation ratios, sieve-based weights (SBWs), and ultrastructural observations of planktic foraminiferal tests. Our bulk CaCO3-based analyses and Globorotalia inflata SBWs revealed three major CaCO3 dissolution events during colder stages of MIS 5d and 5b and at the MIS 5/4 boundary that are traceable events in the eastern South Pacific along the Chilean margin and in the Drake Passage. Furthermore, CaCO3 burial exhibited pronounced glacial/interglacial fluctuations, with almost no burial during glacials (MIS 6, 4, 3, and 2) and recovery during interglacials (MIS 5e and 1) and early glacials (MIS 5d–a). This pattern agrees with previous observations over a wide area of the Southern Ocean, except in the deep Cape Basin > 4600 m in the South Atlantic Ocean. Considering that our sites were located upstream of the Drake Passage, the Circumpolar Deep Water, which was influenced by carbon-rich Pacific Deep Water, likely propagated from the subantarctic eastern Pacific to the South Atlantic at least at depths of ~ 3000 to ~ 4000 m and decreased CaCO3 burial during glacials. These findings supported the importance of carbonate compensation in the Southern Ocean for the carbon cycle on the glacial/interglacial timescale. [ABSTRACT FROM AUTHOR]

In the Mediterranean Sea, organic carbon-rich sapropels have been deposited periodically over the last fifteen million years. Some sapropels are characterized by high contents of the mat-forming, planktonic diatom Pseudosolenia calcar-avis and the planktonic diatom Thalassionema nitzschioides as a result of their mass sinking in autumn (the so-called "fall dump"). The present study shows that fall dump events also occurred in the brackish Baltic Sea around 6300–5800 calibrated years before present (cal yr BP; present = AD 1950). In sediments from the northern Baltic Sea these events are evidenced by high contents of a lipid specific for P. calcar-avis (a C25:2 highly branched isoprenoid alkene) and corroborated by fossil remains of both P. calcar-avis and T. nitzschioides. A biomarker index based on long-chain alkyl diol lipids indicates that mat-forming Proboscia diatoms were also present. High contents of calanoid resting eggs further suggest that the copepod population was stressed being unable to feed on such large diatoms. The fall dump events occurred during a complete stratification of the water column and euxinic conditions, as reflected by the redox-sensitive trace metal uranium, allowing upward diffusion of nutrients and the growth of rhizosolenid mat-forming diatoms in a deep chlorophyll maximum. Synchronous remains of P. calcar-avis in the central Baltic Sea further suggest that these events occurred on a basin-wide scale. [ABSTRACT FROM AUTHOR]