Dirk Kroon

African monsoon variability during the previous interglacial maximum

Abstract Little is known about centennial- to millennial-scale climate variability during interglacial times, other than the Holocene. We here present high-resolution evidence from anoxic (unbioturbated) sediments in the eastern Mediterranean Sea that demonstrates a sustained ∼800-yr climate disturbance in the monsoonal latitudes during the Eemian interglacial maximum (∼125 ka BP). Results imply that before and after this event, the Intertropical Convergence Zone (ITCZ) penetrated sufficiently beyond the central Saharan watershed (∼21°N) during the summer monsoon to fuel flooding into the Mediterranean along the wider North African margin, through fossil river/wadi systems that to date have been considered only within a Holocene context. Relaxation in the ITCZ penetration during the intra-Eemian event curtailed this flux, but flow from the Nile – with its vast catchment area – was not affected. Previous work suggests a concomitant Eurasian cooling event, with intensified impact of the higher-latitude climate on the Mediterranean basin. The combined signals are very similar to those described for the Holocene cooling event around 8 ka BP. The apparent type of concurrent changes in the monsoon and higher-latitude climate may reflect a fundamental mechanism for variability in the transfer of energy (latent heat) between the tropics and higher latitudes.

Monsoon related variations in Zaire (Congo) sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial Atlantic during the last 200,000 years

Two sediment cores from the Zaire Fan and the Angola Margin have been investigated for their composition of terrigenous and biogenic constituents, respectively. For the late Quaternary, kaolinite/feldspar ratios and variations of terrigenous element ratios of Zr, Ti, K, Rb, and Al reveal that the composition of Zaire River sediment load has fluctuated in tune with precessional variations of boreal summer insolation. In particular, the correspondence of high kaolinite/feldspar and Al/K ratios with low-latitude insolation maxima strongly corroborates the assumption that west African monsoonal precipitation and chemical weathering was enhanced during periods of increased central African heating. The most striking feauture observed is that opal accumulation has been 2 to 10 times higher in Zaire Fan sediments than outside in continental margin sediments off Angola, although biogenic Ba and Corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the Zaire River plume and off Angola. From this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the Zaire River plume than in the oceanic upwelling regime farther to the south off Angola over the last 200,000 years. The pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the Zaire River to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent, while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold, more arid glacial climate conditions. We presume that paleoproductivity off the Zaire was controlled by the following two sources of nutrients: (1) marine nutrients including nitrate and phosphate as well as the uptake of Ba on particulate Corg caused by upwelling, and (2) silica, mainly delivered by the Zaire River dissolved silicate. Hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical Atlantic at a millenial timescale.

The isotopic signature of planktonic foraminifera from NE Atlantic surface sediments: implications for the reconstruction of past oceanic conditions

The stable isotope compositions of the planktonic foraminifera Globigerina bulloides, Globigerinoides ruber (white and pink varieties), Globigerinoides trilobus, Globorotalia inflata and Globorotalia truncatulinoides (right‐ and left‐coiling types) were examined as recorders of North Atlantic surface water properties based on 40 box‐core surface sediments between 60° and 30°N. While G. ruber (white and pink varieties) and G. trilobus mainly reflect summer surface water conditions in their oxygen isotope composition, G. bulloides reflects temperatures of the northward‐migrating spring bloom, February–March in the south to May–June in the north. Our data show that G. bulloides cannot be regarded as an indicator for summer temperatures as deduced from Duplessy et al.’s data. Gt. inflata and Gt. truncatulinoides (right‐ and left‐coiling) build their shells in the coldest waters compared with the other species and reflect temperatures between 100 and 400 m water depth. The difference in oxygen isotope composition between G. bulloides and G. inflata serves as a proxy for water mass stratification. G. bulloides is the only species that gives a distinct pattern in its carbon isotopic composition showing a high correlation with the surface water phosphate values along the transect and may serve as a proxy for palaeonutrients and/or productivity.

Reconstructing past planktic foraminiferal habitats using stable isotope data: a case history for Mediterranean sapropel S5

Abstract A high-resolution stable O and C isotope study is undertaken on all planktic foraminiferal species that are reasonably continuous through an Eemian sapropel S5 from the western side of the eastern Mediterranean. The data are considered within a context of high-resolution isotope records for two further S5 sapropels from the central and easternmost sectors of the basin, alkenone-based sea surface temperature records for all three sapropels, and planktic foraminiferal abundance records for the same sample sets through all three sapropels. Results are compared with similar data for Holocene sapropel S1. The adopted approach allows distinction between species that are most suitable to assess overall changes in the climatic/hydrographic state of the basin, including depth-related differentiations and the main seasonal developments, and species that are most affected by variable biological controls or local/regional and transient physico–chemical forcings. It is found that a-priori assumptions about certain species’ palaeohabitats, based on modern habitat observations, may become biased when non-analogue conditions develop. In the case of Mediterranean sapropel S5, these consisted of enhanced freshwater dilution, elevated productivity, shoaling of the pycnocline between intermediate and surface waters, and stagnation of the subsurface circulation. Under these conditions, some species are found to ‘shift’ into habitat settings that differ considerably from those occupied today. The present multiple-species approach can identify such ‘anomalous responses’, and thus offers a sound background for further shell-chemistry investigations and quantitative interpretation of the isotopic profiles. We capitalise on the latter potential, and offer the first quantitative estimates of monsoon flooding into the Mediterranean during the deposition of Eemian sapropel S5.

Rapid palaeoceanographic changes in the Benguela Upwelling System for the last 160,000 years as indicated by abundances of planktonic foraminifera

Abstract Two sediment cores retrieved from the continental slope in the Benguela Upwelling System, GeoB 1706 (19°33.7′S 11°10.5′E) and GeoB 1711 (23°18.9′S, 12°22.6′E), reveal striking variations in planktonic foraminiferal abundances during the last 160,000 years. These fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off Namibia. Four species make up over 95% of the variation within the core, and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages. The fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment. The cold-water planktonic foraminifer, Neogloboquadrina pachyderma sinistral [N. pachyderma (s)], dominates the modern-day, coastal upwelling centres, and Neogloboquadrina pachyderma dextral and Globigerina bulloides characterize the fringes of the upwelling cells. Globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional, reduced nutrient levels of the subtropical waters. In the fossil record, episodes of high N. pachyderma (s) abundances are interpreted as evidence of increased upwelling intensity, and the associated increase in nutrients. The N. pachyderma (s) record suggests temporal shifts in the intensity of upwelling, and corresponding trophic domains, that do not follow the typical glacial-interglacial pattern. Periods of high N. pachyderma (s) abundance describe rapid, discrete events dominating isotope stages 3 and 2. The timing of these events correlates to the temporal shifts of the Angola-Benguela Front (Jansen et al., 1997) situated to the north of the Walvis Ridge. Absence of high abundances of N. pachyderma (s) from the continental slope of the southern Cape Basin indicates that Southern Ocean surface water advection has not exerted a major influence on the Benguela Current System. The coincidence of increased upwelling intensity with the movement of the Angola-Benguela Front can be interpreted mainly by changes in strength and zonality of the trade wind system.

A 300 kyr high-resolution aridity record of the North African continent

New oxygen isotope data and a high-resolution carbonate record of a core from the northwest African margin are presented, in combination with geochemical (Zr/Rb ratio, Al, Ba, organic carbon, and biogenic opal fluxes) and sedimentological (median grain size) parameters, to show variations in African continental aridity, wind strength, and upwelling productivity. The carbonate record is diluted by the supply of aeolian dust from the arid Saharan region of north Africa and is seen to vary with a characteristic sawtooth pattern indicating rapid increases in dust flux followed by gradual declines. Spectral analysis of the carbonate profile and comparison with SPECMAP (stacked oxygen isotope) and ETP (solar radiation) profiles shows that the precessional (23 kyr) insolation-induced African climate system is decoupled from global (ice volume) climate changes operating on a 100 kyr frequency. Detailed studies of glacial terminations I, II, and III confirm the presence of rapid, short timescale climatic variations (in aridity).

Stepwise Holocene aridification in NE Africa deduced from dust-borne radiogenic isotope records

Abstract Transfer of tropical heat to higher latitudes is the major driving force of the Earth’s climate. Consequently, sediments in regions to the north and south of the tropics potentially retain an archive of past major climate reconfigurations. The climate of one such region, around the Arabian Sea, sensitively depends on the coupled Asian and African monsoons that also control the dust transport. Here, we use the Sr–Nd isotope ratios of the dust fraction from Core 905 (Arabian Sea off Somalia), as a novel tool to deduce the Holocene weathering history of the Horn of Africa with emphasis on the climate transition that took place from a wet early to a dry late Holocene. The highly variable Sr isotope ratios are interpreted to reflect mainly changes in the evaporation/precipitation balance over NE Africa whilst the Nd isotope measurements record no significant variations and point to a prevailing NE African dust source. The Sr isotope record shows that the first aridification step occurred at 8.5 kyr BP followed by an unstable transitional period up to 6 kyr BP, characterized by decadal-scale high-amplitude variations in the evaporation/precipitation balance. A second aridification step began at 6 kyr BP and ceased at 3.8 kyr BP when modern-day dry climate was established. The combined Sr and Nd isotope records probably reflect north–south shifts of the Intertropical Convergence Zone controlling the evaporation/precipitation balance over NE Africa.

Deglacial surface circulation changes in the northeastern Atlantic: Temperature and salinity records off NW Scotland on a century scale

Sea surface temperature and salinity estimates reconstructed from a core collected on the Barra Fan, northwest Scotland (56°43′N, 09°19′W; water depth 1320 m), show a series of rapid oscillations during the last deglacial period that are very similar to those observed in the δ18O records from Greenland ice cores. These records indicate that the transport of heat and salt toward the Nordic Seas was highest during the Bolling period. This “superconveyor” weakened after the Bolling, probably as a consequence of increased meltwater flux reducing the oceanic salt content, as suggested by the Barbados sea-level record. Evidence for a phase of ice rafting during the Allerod is presented for the first time from this latitude in the northeast Atlantic. The Younger Dryas stadial, resolved here at a century/decadal scale, is characterized by very rapid oscillations in temperature and salinity, indicating that warm, relatively saline waters repeatedly displaced cool polar waters at this latitude. These observations attest to the inherent instability of the deglacial climate system.

Relevance of aquatic environments for hominins: a case study from Trinil (Java, Indonesia).

Knowledge about dietary niche is key to understanding hominin evolution, since diet influences body proportions, brain size, cognition, and habitat preference. In this study we provide ecological context for the current debate on modernity (or not) of aquatic resource exploitation by hominins. We use the Homo erectus site of Trinil as a case study to investigate how research questions on possible dietary relevance of aquatic environments can be addressed. Faunal and geochemical analysis of aquatic fossils from Trinil Hauptknochenschicht (HK) fauna demonstrate that Trinil at approximately 1.5Ma contained near-coastal rivers, lakes, swamp forests, lagoons, and marshes with minor marine influence, laterally grading into grasslands. Trinil HK environments yielded at least eleven edible mollusc species and four edible fish species that could be procured with no or minimal technology. We demonstrate that, from an ecological point of view, the default assumption should be that omnivorous hominins in coastal habitats with catchable aquatic fauna could have consumed aquatic resources. The hypothesis of aquatic exploitation can be tested with taphonomic analysis of aquatic fossils associated with hominin fossils. We show that midden-like characteristics of large bivalve shell assemblages containing Pseudodon and Elongaria from Trinil HK indicate deliberate collection by a selective agent, possibly hominin.

Holocene palaeolimnology of Kajemarum Oasis, Northern Nigeria: An isotopic study of ostracodes, bulk carbonate and organic carbon

A high-resolution 5500 year palaeolimnological record from Kajemarum Oasis, a closed basin in the Manga Grasslands of northern Nigeria, provides evidence of environmental change in Subsaharan Africa during the Holocene. Palaeohydrological variations, mainly changes in the balance between precipitation and evaporation, are recorded by stable oxygen isotope ratios in bulk carbonate and ostracod calcite and by the Sr/Ca ratio in ostracode shells. Variations in the carbon isotope ratios in carbonates indicate changes in primary productivity in the lake, whereas the carbon isotope composition of organic carbon reflects ecophysiological processes within the lake and its surrounding catchment. Results indicate that there have been marked environmental changes in the Manga Grasslands over the last 5500 years. A more variable climate set in around 1500 cal. a BP. A prolonged drought between 1200 and 1000 cal. a BP, with reduced aquatic productivity, was followed by a switch to a wetter, but still unstable climate: moist conditions prevailed during the Little Ice Age. The results indicate that drought has affected the Sahel episodically over the last 1500 years and is not solely a twentieth-century phenomenon.