Holger Kuhlmann

Forcing of wet phases in southeast Africa over the past 17,000 years

Intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast Africa since the Last Glacial Maximum, about 20,000 years ago. In particular, the relative importance of atmospheric and oceanic processes is not firmly established. Southward shifts of the intertropical convergence zone (ITCZ) driven by high-latitude climate changes have been suggested as a primary forcing, whereas other studies infer a predominant influence of Indian Ocean sea surface temperatures on regional rainfall changes. To address this question, a continuous record representing an integrated signal of regional climate variability is required, but has until now been missing. Here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro-climatology in southeast Africa during rapid climate changes over the past 17,000 years. Our results are based on a reconstruction of precipitation and river discharge changes, as recorded in a marine sediment core off the mouth of the Zambezi River, near the southern boundary of the modern seasonal ITCZ migration. Indian Ocean sea surface temperatures did not exert a primary control over southeast African hydrologic variability. Instead, phases of high precipitation and terrestrial discharge occurred when the ITCZ was forced southwards during Northern Hemisphere cold events, such as Heinrich stadial 1 (around 16,000 years ago) and the Younger Dryas (around 12,000 years ago), or when local summer insolation was high in the late Holocene, that is, during the past 4,000 years.

Upwelling intensity and filament activity off Morocco during the last 250,000 years

Abstract The high-productive upwelling area off Morocco is part of one of the four major trade-wind driven continental margin upwelling zones in the world oceans. While coastal upwelling occurs mostly on the shelf, biogenic particles derived from upwelling are deposited mostly at the upper continental slope. Nutrient-rich coastal water is transported within the Cape Ghir filament region at 30°N up to several hundreds of kilometers offshore. Both upwelling intensity and filament activity are dependent on the strength of the summer Trades. This study is aimed to reconstruct changes in trade wind intensity over the last 250,000 years by the analysis of the productivity signal contained in the sedimentary biogenic particles of the continental slope and beneath the Cape Ghir filament. Detailed geochemical and geophysical analyses (TOC, carbonate, C/N, δ13Corg, δ15N, δ13C of benthic foraminifera, δ18O of benthic and planktic foraminifera, magnetic susceptibility) have been carried out at two sites on the upper continental slope and one site located further offshore influenced by the Cape Ghir filament. A second offshore site south of the filament was analyzed (TOC, magnetic susceptibility) to distinguish the productivity signal related to the filament signal from the general offshore variability. Higher productivity during glacial times was observed at all four sites. However, the variability of productivity during glacial times was remarkably different at the filament-influenced site compared to the upwelling-influenced continental slope sites. In addition to climate-related changes in upwelling intensity, zonal shifts of the upwelling area due to sea-level changes have impacted the sedimentary productivity record, especially at the continental slope sites. By comparison with other proxies related to the strength and direction of the prevailing winds (Si/Al ratio as grain-size indicator, pollen) the productivity record at the filament-influenced site reflects mainly changes in trade-wind intensity. Our reconstruction reveals that especially during glacial times trade-wind intensity was increased and showed a strong variability with frequencies related to precession.

Storminess control over African dust input to the Moroccan Atlantic margin (NW Africa) at the time of maxima boreal summer insolation: a record of the last 220 kyr

Abstract Precessional forcing of dust flux from Northwest Africa to the Atlantic during the last 220 kyr is recognised through changes in physical and chemical sediment properties (sediment colour reflectance, magnetic susceptibility, X-ray fluorescence element intensity) in a core from the Moroccan margin. Sediment colour reflectance, magnetic susceptibility, and Ca and Fe element intensity changes are thought to be driven by dust input from North Africa. Spectral analysis of the sediment properties records displays a dominant periodicity in the 1/23 kyr −1 frequency band that is associated with the Earth’s orbital precession. Peaks in terrigenous supply match precessional minima. This suggests a close link between maxima of boreal summer solar radiation, monsoon activity, and dust generation. Enhanced precession-driven solar radiation would have caused increased seasonal temperature contrasts, which amplified atmospheric turbulence and stimulated storminess. Such a scenario is similar to today’s summer conditions, when frequent storms cause distinct dust plumes to form in the area and transport fine terrigenous material to the adjacent North Atlantic Ocean.

Productivity response in the North Canary Basin to climate changes during the last 250 000 yr: a multi-proxy approach

We present results from the investigation of the primary productivity record over the last 250 kyr in the North Canary Basin (30°N) off Northwest Africa. Two distinct productive systems interfere in this area: the oligotrophic open ocean and the upwelling filament off Cape Ghir, that occasionally carries offshore cool nutrient-rich waters. The following geochemical and micropaleontological paleoproductivity proxies have been used in our study: calcium carbonate, barium excess (Baexcess), total organic carbon (TOC) and diatoms. Time series analysis of these proxies indicates that paleoproductivity in the North Canary Basin underwent important changes following precession and eccentricity cycles. While the precessional signal appears to be mainly related to trade wind strength, superimposed peaks in Baexcess, TOC and diatom records point to large productivity events at Terminations I, II and III. Lowering of the North Atlantic sea surface temperatures by melt water discharges which in turn strengthened the Azores high-pressure center and increased trade wind velocities is postulated as the mechanism to explain the enhancement of the coastal upwelling and associated filaments at terminations. Additionally, the Canary Current may play a role in transmitting cold melt waters and nutrients from higher latitudes to the North Canary Basin.

Weakening and strengthening of the Indian monsoon during Heinrich events and Dansgaard-Oeschger oscillations

The Dansgaard-Oeschger oscillations and Heinrich events described in North Atlantic sediments and Greenland ice are expressed in the climate of the tropics, for example, as documented in Arabian Sea sediments. Given the strength of this teleconnection, we seek to reconstruct its range of environmental impacts. We present geochemical and sedimentological data from core SO130-289KL from the Indus submarine slope spanning the last similar to 80 kyr. Elemental and grain size analyses consistently indicate that interstadials are characterized by an increased contribution of fluvial suspension from the Indus River. In contrast, stadials are characterized by an increased contribution of aeolian dust from the Arabian Peninsula. Decadal-scale shifts at climate transitions, such as onsets of interstadials, were coeval with changes in productivity-related proxies. Heinrich events stand out as especially dry and dusty events, indicating a dramatically weakened Indian summer monsoon, potentially increased winter monsoon circulation, and increased aridity on the Arabian Peninsula. This finding is consistent with other paleoclimate evidence for continental aridity in the northern tropics during these events. Our results strengthen the evidence that circum-North Atlantic temperature variations translate to hydrological shifts in the tropics, with major impacts on regional environmental conditions such as rainfall, river discharge, aeolian dust transport, and ocean margin anoxia. Key Points Intensity of Indian monsoon is traced with geochemical and grain size analyses of sediments Fluvial versus aeolian sediment input to Arabian Sea mimics DO oscillations During Heinrich events the Indian monsoon weakened distinctly

Anatomy of Heinrich Layer 1 and its role in the last deglaciation

This research used data acquired at the XRF Core Scanner Lab at the MARUM–Center for Marine Environmental Sciences, University of Bremen, Germany. This research used samples provided by the International Ocean Discovery Program (IODP). Funding for this research was provided by the UK Natural Environmental Research Council (NERC) to Hodell. The NERC Radiocarbon Facility supported two radiocarbon dates, and Wally Broecker generously supported the remainder with funding from the Comer Family Foundation. Research by Rodriguez-Tovar and Dorador was financed by Project CGL2015-66835-P. B.M. acknowledges support from the CSIC-Ramon y Cajal postdoctoral programme RYC-2013-14073. J.F.E. would like to acknowledge funding under ERC Advanced grant 320750- Nanopaleomagnetism.

Holocene tropical western Indian Ocean sea surface temperatures in covariation with climatic changes in the Indonesian region

The sea surface temperature (SST) of the tropical Indian Ocean is a major component of global climate teleconnections. While the Holocene SST history is documented for regions affected by the Indian and Arabian monsoons, data from the near-equatorial western Indian Ocean are sparse. Reconstructing past zonal and meridional SST gradients requires additional information on past temperatures from the western boundary current region. We present a unique record of Holocene SST and thermocline depth variations in the tropical western Indian Ocean as documented in foraminiferal Mg/Ca ratios and δ18O from a sediment core off northern Tanzania. For Mg/Ca and thermocline δ18O, most variance is concentrated in the centennial to bicentennial periodicity band. On the millennial time scale, an early to mid-Holocene (~7.8–5.6 ka) warm phase is followed by a temperature drop by up to 2°C, leading to a mid-Holocene cool interval (5.6–4.2 ka). The shift is accompanied by an initial reduction in the difference between surface and thermocline foraminiferal δ18O, consistent with the thickening of the mixed layer and suggestions of a strengthened Walker circulation. However, we cannot confirm the expected enhanced zonal SST gradient, as the cooling of similar magnitude had previously been found in SSTs from the upwelling region off Sumatra and in Flores air temperatures. The SST pattern probably reflects the tropical Indian Ocean expression of a large-scale climate anomaly rather than a positive Indian Ocean Dipole-like mean state.