Maarten A. Prins

A 300-kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic

Abstract The terrigenous fraction of sediments recovered from Walvis Ridge, SE Atlantic Ocean, reveals a history of southwestern African climate of the last 300 kyr. End-member modelling of a data set of grain-size distributions (n=428) results in three end members. The two coarsest end members are interpreted as eolian dust, the third end member as hemipelagic mud. The ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the SE trade winds. Trade winds were intensified during glacials compared to interglacials. Changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern African aridity. Late Quaternary southwestern African climate was relatively arid during the interglacial stages and relatively humid during the glacial stages, owing to meridional shifts in the atmospheric circulation system. During glacials the polar front shifted equatorward, resulting in a northward displacement of the zone of westerlies, causing increased rainfall in southwestern Africa. The equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient, leading to enhanced atmospheric circulation and increased trade-wind intensity.

Muddled or mixed? Inferring palaeoclimate from size distributions of deep-sea clastics

Abstract One of the outstanding problems of palaeoclimate reconstruction from physico-chemical properties of terrigenous deep-sea sediments stems from the fact that most basin fills are mixtures of sediment populations derived from different sources and transported to the site of deposition by different mechanisms. Conventional approaches to palaeoclimate reconstruction from deep-sea sediments do not distinguish between provenance and dispersal-related variations, and therefore often fail to recognise the true significance of variations in sediment properties. We formulate a set of requirements that each proposed palaeo-environmental indicator should fulfil, and focus on the intrinsic coupling between grain size and chemical composition. A critical review of past achievements in grain-size analysis serves to introduce a conceptual model of spatio-temporal grain-size variation based on dynamic populations. Each dynamic population results from a characteristic combination of production and transport mechanisms that corresponds to a distinct subpopulation in the data analysed. The mathematical–statistical representation of the conceptual model can be obtained by means of the end-member-modelling algorithm EMMA. Applications of the model to several ocean basins are discussed, as well as methods to examine the validity of grain-size-based palaeoclimate reconstructions. Palaeoclimate reconstructions of a high- and low-latitude basin illustrate the common degree of complexity of deep-sea grain-size records.

Millennial-scale glacial variability versus Holocene stability: changes in planktic and benthic foraminifera faunas and ocean circulation in the North Atlantic during the last 60 000 years

Two piston cores, DS97-2P from the Reykjanes Ridge in the central North Atlantic Ocean (1685 m water depth) and ENAM33 from southwest of the Faeroe Islands in the NE Atlantic (1217 m water depth), have been investigated for their planktic and benthic foraminiferal content. DS97-2P is situated near the Subarctic Front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the Holocene. The productivity shows a clear decrease from an early Holocene maximum to a late Holocene minimum. Coeval changes in the benthic faunas indicate that the food supply changed from large, irregular pulses during the early Holocene to a more sustained flux during the late Holocene. Presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer. Another feature of the late Holocene is an increasing instability of the North Atlantic thermohaline circulation regime. Nevertheless, the changes in faunal composition and productivity during the Holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial. The glacial shifts were on a millennial time scale and correlate with the interstadial-stadial phases of the Dansgaard-Oeschger cycles in the Greenland ice cores. The faunas of the warm interstadial phases resembled the Holocene faunas, and both surface and bottom productivity was high. The faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the Nordic seas and generation of North Atlantic Deep Water. The planktic faunas during the cold stadials and Heinrich events were completely dominated by the polar species Neogloboquadrina pachyderma s, and surface conditions were cold and the productivity low. The benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content. The water column was probably stratified with low saline, cold surface water overlying poorly aerated, intermediate water masses.

Late Quaternary sedimentary processes and ocean circulation changes at the Southeast Greenland margin

A study has been made of late Quaternary depositional processes and bottom current activity on the Southeast Greenland margin, using seismic, sub-bottom profiling and deep-tow side-scan sonar data as well as sediment core information. The seabed data demonstrate the occurrence of strong, southerly bottom currents prevailing on the slope and rise. Well-defined longitudinal bedforms indicate maximum mean near-bottom current velocities of up to at least 1.0 m/s at the depth stratum of Labrador Sea water (800–1500 m). Similarly strong currents occur in Denmark Strait overflow water (DSOW) at the base of the slope, whereas more basinward the maximum DSOW flow speed is lower. Iceberg plow marks were found down to about 700 m water depth. Both at the shelf edge and on the lower slope and rise the seafloor morphology is indicative of downslope sediment transport and mass flow deposition, which is concluded to be a typically glacial feature. After generally more sluggish deep-water circulation during the last glacial maximum, DSOW basin ventilation was re-established shortly before 13.3 ka. On the shelf, in front of the retreating Greenland ice margin, permanent or semi-permanent sea ice conditions prevailed until about 12.5 ka. At that time increased Irminger Current activity had resulted in warming, and East Greenland Current (EGC)-controlled iceberg drift increased. No evidence was found for a return to extreme glacial conditions or a ceasing of DSOW flow during the Younger Dryas. Abundant coarse IRD collected at greater water depth from shallow sub-seabed strata has a provenance mainly in the northern part of East Greenland (68–73°N), which demonstrates the existence of a pre-Holocene EGC system initially extending to beyond the shelf edge.

Glacial and interglacial eolian dust dispersal patterns across the Chinese Loess Plateau inferred from decomposed loess grain‐size records

Previous studies have indicated that a genetically meaningful decomposition (unmixing) of loess grain-size distributions can be accomplished with the end-member modeling algorithm EMMA. The independent decomposition of two series of loess grain-size records from the NE Tibetan Plateau and Loess Plateau spanning the last glacial-interglacial cycle indicates that the two data sets are described by very similar mixing models. The average mixing model presented here is regarded as representative for the vast loess region in northern China and allows quantification of the contribution of three loess components to the loess grain-size distributions. A genetic interpretation and the paleoclimatic significance of the average mixing model have been provided by comparison of the modeled loess components with modern dust samples in terms of their grain-size distribution and flux rates, and by the distribution patterns of the loess components across the Loess Plateau reconstructed for the last two glacial-interglacial cycles. The sandy and silty loess components represent the coarse dust fraction supplied by saltation and short-term suspension processes over the proximal part of the Loess Plateau during major dust outbreaks in spring and early summer. The low-level winter monsoon (northwesterly wind system) is the likely transporting agent for these dust events. A clayey loess component represents the fine dust component supplied over the entire Loess Plateau by long-term suspension processes during major dust outbreaks and as part of a background supply system. The clayey loess component in the glacial loess deposits is dominantly supplied during major dust outbreaks by the northwesterly winter monsoon, whereas the clayey loess component in the interglacial paleosols is mainly supplied by non-dust-storm processes, possibly with a significant contribution by the high-level subtropical jet stream (westerly winds).

Correcting for the influence of ice‐rafted detritus on grain size‐based paleocurrent speed estimates

The grain size of deep-sea sediments provides an apparently simple proxy for current speed. However, grain size-based proxies may be ambiguous when the size distribution reflects a combination of processes, with current sorting only one of them. In particular, such sediment mixing hinders reconstruction of deep circulation changes associated with ice-rafting events in the glacial North Atlantic because variable ice-rafted detritus (IRD) input may falsely suggest current speed changes. Inverse modelling has been suggested as a way to overcome this problem. However, this approach requires high-precision size measurements that register small changes in the size distribution. Here we show that such data can be obtained using electro sensing and laser diffraction techniques, despite issues previously raised on the low precision of electro sensing methods and potential grain shape effects on laser diffraction. Down-core size patterns obtained from a sediment core from the North Atlantic are similar for both techniques, reinforcing the conclusion that both techniques yield comparable results. However, IRD input leads to a coarsening that spuriously suggests faster current speed. We show that this IRD influence can be accounted for using inverse modelling as long as wide size spectra are taken into account. This yields current speed variations that are in agreement with other proxies. Our experiments thus show that for current speed reconstruction the choice of instrument is subordinate to a proper recognition of the various processes that determine the size distribution and that by using inverse modelling meaningful current speed reconstructions can be obtained from mixed sediments.

Soil archives of a Fluvisol: subsurface analysis and soil history of the medieval city centre of Vlaardingen, the Netherlands – an integral approach

(1) VU University Amsterdam, Dept. of Archaeology, Arts, Amsterdam, Netherlands (s.j.kluiving@vu.nl), (2) City of Vlaardingen, VLAK (Archaeology Dept., Hoflaan 43, 3134 AC Vlaardingen, The Netherlands, (3) Arnicon, Archeomedia 2908 LJ Capelle aan den IJssel, The Netherlands, (4) VU University Amsterdam, Faculty of Earth and Life Sciences, Department of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands, (5) Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam,

Columbus’ environmental impact in the New World: Land use change in the Yaque River valley, Dominican Republic

Columbus’ arrival in the New World in AD 1492 on the northern coast of Hispaniola was followed by a suite of changes in land-use. We reconstruct environmental change from a 225-cm-long sediment core from site Los Indios from an abandoned and sediment-filled meander of the Yaque River, Cibao Valley, northeastern Dominican Republic. The sediment record starts ca. AD 195 (ca. 1755 cal. yr BP) and the history of the meander infill was monitored by changing grain size distributions, organic matter concentration and pollen from wetland plants. From ca. AD 200 to ca. AD 1525, the pollen record indicates a diverse forest assemblage; however, the presence of pollen from potential crop plants suggest nearby small-scale subsistence crop cultivation. More abundant charcoal after ca. AD 1410 shows Amerindians increasingly used fire. The record of grain size distributions shows that the meander was temporarily part of a low energetic drainage system in which bedload and suspended sediments accumulated. After European colonization of Hispaniola increasing spores of coprophilous fungi evidence that Europeans had introduced during the first decades of colonization cattle in the Cibao Valley which gradually resulted in more open forest. The charcoal record around ca. AD 1650 reflects intensive forest clearing, suggesting that small-scale Pre-Colonial practice of crop cultivation became replaced by large-scale agriculture on the moist and nutrient rich soils along the Yaque River. Further deforestation and signals of erosion suggest that the population of colonists and introduced enslaved labour force must have increased rapidly. After ca. AD 1740 charcoal influx decreased suggesting that last deforestation activities used selective cutting to produce fire wood and timber for construction, rather than burning forest in situ. Two centuries after European colonization, by the 18th century, land-use within the Cibao Valley had become a balance between substantial livestock and crop cultivation (pollen grains have evidenced cereals, maize, and potentially also sugar cane, amaranthaceous crops and tobacco). After ca. AD 1950, swamp vegetation of Typha and Cyperaceae decreased, pointing to an almost fully terrestrialized meander with only few bodies of standing water, reflecting the present-day setting. This multiproxy reconstruction of anthropogenic environmental change shows a clear differentiation between an immediate introduction of livestock and after some 150 years the development of a European style agriculture, providing a context for archaeological investigations.