Abrupt shifts of the Sahara-Sahel boundary during Heinrich stadials demonstrated on a sediment core transect

Relict dune fields that are found as far south as 14° N in the modern-day African Sahel are testament to equatorward expansions of the Sahara desert during the Late Pleistocene. However, the discontinuous nature of dune records means that abrupt millennial-timescale climate events are not always resolved. High-resolution marine core studies have identified Heinrich stadials as the dustiest periods of the last glacial in West Africa although the spatial evolution of dust export on millennial timescales has so far not been investigated. We use the major-element composition of four high-resolution marine sediment cores to reconstruct the spatial extent of Saharan-dust versus river-sediment input to the continental margin from West Africa over the last 60 ka. This allows us to map the position of the sediment composition corresponding to the Sahara-Sahel boundary. Our records indicate that the Sahara-Sahel boundary reached its most southerly position (13° N) during Heinrich stadials and hence suggest that these were the periods when the sand dunes formed at 14° N on the continent. Heinrich stadials are associated with cold North Atlantic sea surface temperatures which appear to have triggered abrupt increases of aridity and wind strength in the Sahel. Our study illustrates the influence of the Atlantic meridional overturning circulation on the position of the Sahara-Sahel boundary and on global atmospheric dust loading.

Isotopic composition of ground ice, ebullition gases and thermokarst lake water, North America

Thermokarst lakes are thought to have been an important source of methane (CH4) during the last deglaciation when atmospheric CH4 concentrations increased rapidly. Here we demonstrate that meltwater from permafrost ice serves as an H source to CH4 production in thermokarst lakes, allowing for region-specific reconstructions of dD-CH4 emissions from Siberian and North American lakes. dD CH4 reflects regionally varying dD values of precipitation incorporated into ground ice at the time of its formation. Late Pleistocene-aged permafrost ground ice was the dominant H source to CH4 production in primary thermokarst lakes, whereas Holocene-aged permafrost ground ice contributed H to CH4 production in later generation lakes. We found that Alaskan thermokarst lake dD-CH4 was higher (-334 ± 17 per mil) than Siberian lake dD-CH4 (-381 ± 18 per mil). Weighted mean dD CH4 values for Beringian lakes ranged from -385 per mil to -382 per mil over the deglacial period. Bottom-up estimates suggest that Beringian thermokarst lakes contributed 15 ± 4 Tg CH4 /yr to the atmosphere during the Younger Dryas and 25 ± 5 Tg CH4 /yr during the Preboreal period. These estimates are supported by independent, top-down isotope mass balance calculations based on ice core dD-CH4 and d13C-CH4 records. Both approaches suggest that thermokarst lakes and boreal wetlands together were important sources of deglacial CH4.

Hydrogen and oxygen isotopes in ground lce of Bykovsky Peninsula, North Siberia

In wide areas of Northern Siberia, glaciers have been absent since the Late Pleistocene. Therefore, ground ice and especially ice wedges are used as archives for paleoclimatic studies. In the present study, carried out on the Bykovsky Peninsula, eastern Lena Delta, we were able to distinguish ice wedges of different genetic units by means of oxygen and hydrogen isotopes. The results obtained by this study on the Ice Complex, a peculiar periglacial phenomenon, allowed the reconstruction of the climate history with a subdivision of a period of very cold winters (60-55 ka), followed by a long stable period of cold winter temperatures (50-24 ka), Between 20 ka and I I ka, climate warming is indicated in stable isotope compositions, most probably after the Late Glacial Maximum. At that time, a change of the marine source of the precipitation from a more humid source to the present North Atlantic source region was assumed. For the Ice Complex, a continuous age-height relationship was established, indicating syngenetic vertical ice wedge growth and sediment accumulation rates of 0.7 m/ky. During the Holocene optimum, ice wedge growth was probably limited due to the extensive formation of lacustrine environments. Holocene ice wedges in thermokarst depressions (alases) and thermoerosional valleys (logs) were formed after climate deterioration from about 4.5 ka until the present. Winter temperatures were warmer at this time as compared to the cooler Pleistocene. Migration of bound water between ice wedges and segregated ice may have altered the isotopic composition of old ice wedges. The presence of ice wedges as diagnostic features for permafrost conditions since 60 ka, implies that a large glacier extending over the Laptev Sea shelf did not exist. For the remote non-glaciated areas of Northern Siberia, ice wedges were established as a powerful climate archive.

Sedimentology on profiles from Lake Prespa, Greece

Hydro-acoustic surveys and coring campaigns at Lake Prespa were carried out between 2007 and 2009. This paper presents hydro-acoustic profiles and provide lithological and chronostratigraphical information from three up to 15.75 m long sediment sequences from the Macedonian side of the lake. The sediment sequences comprise glacial and interglacial sediments likely deposited from the end of Marine Isotope Stage (MIS) 5 to present day. The information implies a distinct change of sedimentation patterns at the Pleistocene/Holocene transition and the establishment of a relatively strong Holocene current system and deposition of channel-related contourite drift in Lake Prespa. Potential causes for the establishment of this current during the Holocene include significant lake level change, reduced winter ice cover, and/or higher aeolian activity.

(Table 1) Age model for ODP Site 175-1077 based on oxygen isotopes of Globigerinoides ruber pink

Palynological records from the Congo fan reveal environmental change in equatorial Africa occurring 1.05 Ma ago, 100 k.y. before the mid-Pleistocene climatic shift at 0.9 Ma. Prior to 1.05 Ma, a glacial-interglacial rhythm is not obvious in the African vegetation variation. Afterwards, Podocarpus spread in the mountains of central Africa mainly during glacials and Congo River discharge decreased. The sequence of vegetation variation associated with the mid-Pleistocene glacials and interglacials differed from that observed during the late Pleistocene. Between 0.9 and 0.6 Ma, interglacials were characterized by warm dry conditions and glacials were characterized by cool humid conditions, while during the past 0.2 Ma glacials were cold and dry and interglacials warm and humid. Our data indicate that before the Northern Hemisphere ice caps dramatically increased in size (0.9-0.6 Ma), low-latitude climate forcing and response in the tropics played an important role in the initiation of 100 k.y. ice-age cycles. During the mid to late Pleistocene, however, the climate conditions in the tropics were increasingly influenced by the glacial-interglacial variations of continental ice sheets.

Geochemical composition and radiocarbon ages of cores from Lago Fugnano, Tierra del Fuego

Recent advances in the chronology and the palaeoclimatic understanding of Antarctic ice core records point towards a larger heterogeneity of latitudinal climate fluctuations than previously thought. Thus, realistic palaeoclimate reconstructions rely in the development of a tight array of well-constrained records with a dense latitudinal coverage. Climatic records from southernmost South America are critical cornerstones to link these Antarctic palaeoclimatic archives with their South American counterparts. At 54° S on the Island of Tierra del Fuego, Lago Fagnano is located in one of the most substantially and extensively glaciated regions of southernmost South America during the Late Pleistocene. This elongated lake is the largest (~110km long) and non-ice covered lake at high southern latitudes. A multi-proxy study of selected cores allows the characterisation of a Holocene sedimentary record. Detailed petrophysical, sedimentological and geochemical studies of a complete lacustrine laminated sequence reveal variations in major and trace elements, as well as organic content, suggesting high variability in environmental conditions. Comparison of these results with other regional records allows the identification of major known late Holocene climatic intervals and the proposal for a time for the onset of the Southern Westerlies in Tierra del Fuego. These results improve our understanding of the forcing mechanisms behind climate change in southernmost Patagonia.

Calcareous nannofossil abundance and datums of sediments from ODP Leg 170 sites

Three Pleistocene, five Pliocene, and thirteen late and middle Miocene calcareous nannofossil datums have been identified in the Leg 170 cored sequences collected from a transect across the Middle America Trench off the Nicoya Peninsula. Although some nannofossil zones could not be delineated, particularly in the Pliocene and upper Miocene, there appears to be a complete or very nearly complete Pleistocene through lower Miocene section at Sites 1039 and 1040. The oldest assemblages, observed at Site 1039 and 1040, are latest early Miocene in age (nannofossil Zone NN4). These assemblages are associated with gabbro intrusions into the basal sediments (one contact metamorphic hornfels sample contains relict nannofossils), indicating an age for the intrusion event of between 15.6 and 18.2 Ma at both Sites 1039 and 1040. Reference Site 1039, located on the Cocos plate, provides the best-preserved sequence of sediments of late Pleistocene to latest early Miocene age. The sediments cored in the prism sections at Sites 1040, 1041, 1042, and 1043 all indicate that the age of nannofossil assemblages in the prism sediments, including the toe, wedge, and apron, are all Pleistocene with a considerable amount of upper Miocene reworking. A period of low sediment accumulation rates (~5.3 m/m.y.) is recorded for Pliocene and upper Miocene sediments at Sites 1039, 1040, and 1043. Pliocene calcareous nannofossil assemblages characteristic of the ~2.5- to 3.75-m.y. time interval (nannofossil Zones NN16 and equivalent nannofossil Subzones CN12b and CN12a) were not resolved at any site. Nannofossil Zones NN15, NN14, NN13, and NN12 (early late Pliocene to early Pliocene) could not be resolved at any site either because of the absence of marker species. Within the Miocene at Sites 1039 and 1040, nannofossil Zones NN10-NN6 were difficult to differentiate because of the absence of several species that define the zonal boundaries. These intervals, where the nannofossil zones have not been resolved or are partially resolved, are primarily composed of carbonate ooze deposited during an ~8.5-m.y. (2.5-11 Ma) low sediment accumulation rate time interval. The absence of many of the marker species is attributed to warmer water conditions during those periods. Many of the same marker species are absent in the sediments recovered from nearby Deep Sea Drilling Project Site 155 in the Panama Basin.

Documentation of sediment cores from the Great Meteor Seamount, North Atlantic

Sedimentological and biostratigraphic investigations of 15 cores (total length: 88 m) from the vicinity of Great Meteor seamount (about 30° N, 28° W) showed that the calcareous ooze are asymmetrically distributed around the seamount and vertically differentiated into two intervals. East and west of the seampunt, the upper "A"-interval is characterized by yellowish-brown sediment colors and bioturbation; ash layers and diatoms are restricted to the eastern cores. On both seamount flanks, the sediment of the lower "B"-interval are white and very rich in CaCO3 with a major fine silt (2-16 µ) mode (mainly coccoliths). Lamination, manganese micronodules, Tertiary foraminifera and discoasters, and small limestone and basalt fragments are typical of the "B"-interval of the eastern cores only. The sediments contain abundant displaced material which was reworked from the upper parts of the seamount. The sedimentation around the seamount is strongly influenced by the kind of displaced material and the intensity of its differentiated dispersal: the sedimentation rates are generally higher on the east than on the west flank /e.g. in "B": 0.9 cm/1000 y in the W; 3.1 cm/1000 y in the E), and lower for the "A" than for the "B"-interval. The lamination is explained by the combination of increased sedimentation rates with a strong input of material poor in organic carbon producing a hostile environment for benthic life. The CaCO3 content of the core is highly influenced by the proportion of displaced bigenous carbonate material (mainly coccoliths). The genuine in-situ conditions of the dissolution facies are only reflected by the minimum CaCO3 values of the cores (CCD = about 5,500 m; first bend in dissolution curve = 4,000 m; ACD = about 3,400 m). The preservation of the total foraminiferal association depends on the proportions of in-situ versus displaced specimens. In greater water depths (stronger dissolution), for example, the preservation can be improved by the admixture of relatively well preserved displaced foraminifera. Carbonate cementation and the formation of manganese micronodules are restricted to microenvironments with locally increased organic carbon contents (e.g. pellets; foraminifera). The ash layers consist of redeposited, silicic volcanic glass of trachytic composition and Mio-Pliocene age; possibly, they can be derived from the upper part of the seamount. Siliceous organisms, especially diatoms, are frequent close to the ash layers and probably also redeposited. Their preservation was favoured by the increase of the SiO2 content in the pore water caused by the silicic volcanic glass. The cores were biostraftsraphically subdivided with the aid of planktonic foraminifera and partly alsococcoliths. In most cases, the biostratigraphically determined cold- and warm sections could be correlated from core to core. Almost all cores do not penetrate the Late Pleistocene. All Tertiary fossils are reworked. In general, the warm/cold boundary W2/C2 corresponds with the lithostratigraphic A/B boundray. Benthonic foraminifera indicate the original site deposition of the displaced material (summit plateau or flanks of the seamount). The asymmetric distribution of the sediments around the seamount east and west of the NE-directed antarctic bottom current (AABW) is explained by the distortion of the streamlines by the Coriolis force; by this process the current velocity is increased west of the seamount and decreased east of it. The different proportion of displaced material within the "A" and "B" interval is explained by changes of the intensity of the oceanic circulation. At the time of "B" the flow of the AABW around the seamount was stronger than during "A"; this can be inferred from the presence of characteristic benthonic foraminifera. The increased oceanic circulation implies an enhanced differentiation of the current velocities, and by that, also of the sedimentation rates, and intensifies the winnowed sediment material was transported downslope by turbid layers into the deep-sea, incorporated into the current system of the AABW, and asymmetrically deposited around the seamount.

Sediment, isotopic and geochemical record during Marine Isotope Stages 29 to 34 of IODP Site 339-U1387

Centennial-to-millennial scale records from IODP Site U1387, drilled during IODP Expedition 339 into the Faro Drift at 558 m water depth, now allow evaluating the climatic history of the upper core of the Mediterranean Outflow (MOW) and of the surface waters in the northern Gulf of Cadiz during the early Pleistocene. This study focuses on the period from Marine Isotope Stage (MIS) 29 to 34, i.e. the interval surrounding extreme interglacial MIS 31. Conditions in the upper MOW reflect obliquity, precession and millennial-scale variations. The benthic d18O signal follows obliquity with the exception of an additional, smaller d18O peak that marks the MIS 32/31 transition. Insolation maxima (precession minima) led to poor ventilation and a sluggish upper MOW core, whereas insolation minima were associated with enhanced ventilation and often also increased bottom current velocity. Millennial-scale periods of colder sea-surface temperatures (SST) were associated with short-term maxima in flow velocity and better ventilation, reminiscent of conditions known from MIS 3. A prominent contourite layer, coinciding with insolation cycle 100, was formed during MIS 31 and represents one of the few contourites developing within an interglacial period. MIS 31 surface water conditions were characterized by an extended period (1065-1091 ka) of warm SST, but SST were not much warmer than during MIS 33. Interglacial to glacial transitions experienced 2 to 3 stadial/interstadial cycles, just like their mid-to-late Pleistocene counterparts. Glacial MIS 30 and 32 recorded periods of extremely cold (< 12°C) SST that in their climatic impact were comparable to the Heinrich events of the mid and late Pleistocene. Glacial MIS 34, on the other hand, was a relative warm glacial period off southern Portugal. Overall, surface water and MOW conditions at Site U1387 show strong congruence with Mediterranean climate, whereas millennial-scale variations are closely linked to North Atlantic circulation changes.

Compositional and isotopic data on core MD84641

The organic carbon isotopic record of the sapropels(S1 and S3-S10) and intercalated marl oozes has been determined in a 12-m piston core from the eastern Mediterranean. The d13C_organic values are systematically lighter (mean=-21.0±0.82 per mil) in all sapropels and heavier (mean=-18.8±1.07 per mil) in the marl oozes. These differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the Mediterranean, there is no relationship between the C_organic/N ratios and the isotopic values, and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter. Temperature effects on isotope fractionation are also discounted because the change in d13C_organic values between glacial and interglacial horizons is in the opposite sense. Diagenesis, which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances, is unlikely to have caused the observed differences because this mechanism would cause an enrichmet in 12C, implying that all values were even heavier originally, and there is no secular trend in the d13C_organic record. The observed differences in d13C_organic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO2. First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13C deficiency in fresh waters. Hence photosynthesis would have produced isotopically lighter organic material. Second, changes in atmospheric pCO2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO2 in the mixed layer; lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton, whereas levels were less limiting during the interglacials. Concentrations of dissolved CO2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO2 to the mixed layer by upwelling, and this could have further lightened the d13C_organic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO2 levels over longer time periods than can be obtained from ice cores.

Sedimentation rate, bulk density, accumulation rate of terrigenous material and grain sizes of ODP Leg 167 sites

The terrigenous mineral fraction of sediments recovered by drilling during Ocean Drilling Program Leg 167 at Sites 1018 and 1020 is used to evaluate changes in the source and transport of fine-grained terrigenous sediment and its relation to regional climates and the paleoceanographic evolution of the California Current system during the late Pleistocene. Preliminary time scales developed by correlation of oxygen isotope stratigraphies with the global SPECMAP record show average linear sedimentation rates in excess of 100 m/m.y., which provide an opportunity for high-resolution studies of terrigenous flux, grain size, and mineralogy. The mass flux of terrigenous minerals at Site 1018 varies from 5 to 30 g/(cm**2 x k.y.) and displays a general trend toward increased flux during glacials. The terrigenous record at Site 1020 shows a similar pattern of increased glacial input, but overall accumulation rates are significantly lower. Spectral analysis demonstrates that most of this variability is concentrated in frequency bands related to orbital cycles of eccentricity, tilt, and precession. Detailed grain-size analysis performed on the isolated terrigenous mineral fraction shows that sediments from Site 1018 are associated with higher energy transport and depositional regimes than those found at Site 1020. Grain-size data are remarkably uniform throughout the last 500 k.y., with no discernible difference observed between glacial and interglacial size distributions within each site. X-ray diffraction analysis of the <2-µm clay component suggests that the deposition of minerals found at Site 1020 is consistent with transport from a southern source during intervals of increased terrigenous input.

Geochemical data and magnetic measurements of IODP Expedition 308, Hole U1319A from the Gulf of Mexico

A 160 m mostly turbiditic late Pleistocene sediment sequence (IODP Expedition 308, Hole U1319A) from the Brazos-Trinity intraslope basin system off Texas was investigated with paleo- and rock magnetic methods. Numerous layers depleted in iron oxides and enriched by the ferrimagnetic iron-sulfide mineral greigite (Fe3S4) were detected by diagnostic magnetic properties. From the distribution of these layers, their stratigraphic context and the present geochemical zonation, we develop two conceptual reaction models of greigite formation in non-steady depositional environments. The "sulfidization model" predicts single or twin greigite layers by incomplete transformation of iron monosulfides with polysulfides around the sulfate methane transition (SMT). The "oxidation model" explains greigite formation by partial oxidation of iron monosulfides near the iron redox boundary during periods of downward shifting oxidation fronts. The stratigraphic record provides evidence that both these greigite formation processes act here at typical depths of about 12-14 mbsf and 3-4 mbsf. Numerous "fossil" greigite layers most likely preserved by rapid upward shifts of the redox zonation denote past SMT and sea floor positions characterized by stagnant hemipelagic sedimentation conditions. Six diagenetic stages from a pristine magnetite-dominated to a fully greigite-dominated magnetic mineralogy were differentiated by combination of various hysteresis and remanence parameters.

Stable isotope and geochemical record of ODP Hole 124-769A

Deep marine late Pleistocene sediments from Ocean Drilling Program Sulu Sea Site 769 contain a high-resolution record of paleoceanographic change in this strongly monsoonal climatic setting in the tropical western Pacific. Detailed time series of planktonic foraminifer (G.ruber; white variety) d18O, d13C, and bulk CaCO3 mass accumulation rate (MAR) were generated, spanning the last 750 k.y. Sedimentation rates in this portion of the record average 8.5 cm/k.y., and vary from 4 to 16 cm/k.y. Cross spectral analysis of the d18O and d13C time-series demonstrate that each contains increased variance at the primary orbital periodicities. The d18O record shows strong variability in the precessional-band and closely correlates with the SPECMAP d18O record and other high-resolution records. The dominance of a 23-k.y cycle in the d18O record agrees with other studies of the monsoon system in the Indian Ocean that have documented the importance of precessional insolation as a monsoon-forcing mechanism. In addition, d13C is strongly coherent, with d18O at a period of 41 k.y (obliquity), suggesting a connection between surface water CO2 chemistry in the Sulu Sea and high- latitude climatic change. The d18O and d13C time-series both contain increased spectral variance at a period of 30 k.y. Although the source of 30-k.y. variability is unknown, other studies have documented late Pleistocene Pacific Oceanographic variability with a period of 30 k.y. Major- and trace-metal analyses were performed on a second, less-detailed sample series to independently assess paleoproductivity changes and bottom-water conditions through time. Glacial periods are generally times of increased calcium carbonate and copper accumulation. The positive association between these independent indicators of paleoproductivity suggests an increase in productivity in the basin during most glacial episodes. Changing bottom-water redox conditions were also assessed using the geochemical data. Low concentrations of molybdenum throughout the record demonstrate that bottom waters at this site were never anoxic during the last 750 k.y. The bioturbated character of the sediments agrees with this interpretation.

Geochemistry and position of turbidites in the Cap Timiris Canyon off Mauritania

This study of sediments from the Cap Timiris Canyon demonstrates that geochemical data can provide reliable age-depth correlation even of highly turbiditic cores and attempts to improve our understanding of how turbidite emplacement is linked to climatic-related sea-level changes. The canyon incises the continental margin off NW Africa and is an active conduit for turbidity currents. In sediment cores from levee and intrachannel sites turbidites make up 6-42% of sediment columns. Age models were fitted to all studied cores by correlating downcore element data to dated reference cores, once turbidite beds had been removed from the dataset. These age models enabled us to determine turbidite emplacement times. The Cap Timiris Canyon has been active at least over the last 245 kyr, with turbidite deposition seemingly linked to stage boundaries and glacial stages. The highly turbiditic core from the intrachannel site postdates to ~15 kyr and comprises Holocene and late Pleistocene sediments. Turbidite deposition at this site was associated especially with the rapid sea-level rise at the Pleistocene/Holocene transition. During the Holocene, turbidity current activity decreased but did not cease.

Radiolarian biostratigraphy of ODP Site 111-677

Well-preserved late Miocene through Pleistocene age radiolarian assemblages were recovered during ODP Leg 111 at Site 677, on the southern flank of the Costa Rica Rift in the eastern equatorial Pacific. Radiolarian "event" biostratigraphy (first and last morphotypic appearances) was established for Holes 677A and 677B using 21 species yielding 24 reliable datum levels. The cold upwelling waters above this site have prevented many typical tropical Pacific stratigraphic radiolarians from being useful age indicators. Biostratigraphic datum levels were assigned absolute ages based on previous work and were used to date the cores. Sedimentation rates varied from 3.7 cm/1000 yr in the late Pleistocene to 6.0 cm/1000 yr in the late Miocene. The age of the oldest sediments at this site is estimated as 5.89-6.37 Ma, which indicates that Site 677 is between magnetic anomalies 3A and 4. A total of 67 taxa were assessed for stratigraphic relevance at this site and are listed in the Appendix. One previously unknown Pliocene radiolarian stratigraphic indicator, Botryostrobus euporus (Ehrenberg), is identified.