Glacial-interglacial Scales Research Articles

High-resolution trace element and stable/radiogenic isotope profiles of late Pleistocene to Holocene speleothems from Dim Cave, SW Turkey

Multiple climate-sensitive trace element/Ca and stable isotope (O and C) profiles derived from Dim Cave speleothems (S-SW Turkey) provide evidence of climatic changes and define a series of palaeohydrological conditions for the period ~10–90kyr. Dim Cave speleothem Mg/Ca, Sr/Ca, U/Ca, Ba/Ca, and Y/Ca ratios demonstrate similar patterns over glacial–interglacial scales, in agreement with δ18O and δ13C records. Three episodes of more positive moisture balance (71–63kyr, 51–40kyr, and 18–10kyr) were observed based on Y/Ca (and to a lesser extent Zr/Ca), 87Sr/86Sr ratios, calcite micromorphology, and growth rates. Increasing concentrations of Y, Zr (and U) and elevated 87Sr/86Sr ratios are attributed to enhanced levels of terrestrial input during these periods. Correlations between δ13C, δ18O and Mg/Ca during 40–18kyr (corresponding with the lowest growth rate of ~0.8mm/kyr), 63–51, and 80–71kyr (relatively low growth rates), as well as co-varying and enhanced Mg/Ca, Sr/Ca, and to a lesser extent Ba/Ca, ratios point to the prior calcite precipitation, wall rock interaction, and preferential dolomite dissolution over calcite in the host dolomitic limestone during these periods. This relationship suggests that water–rock interactions are maximised during episodes of slower drip rates of water through the karst under drier conditions. Chondrite-normalised rare earth element and yttrium (REY) patterns of the stalagmites reveal seawater signatures closely linked to the dolomitic limestone. Excluding the aragonite formation during ~80–75kyr, which is an autogenic effect, trace element/Ca ratios appear to respond to millennial scale global cooling periods such as Heinrich events.

On the timing and forcing mechanisms of late Pleistocene glacial terminations: Insights from a new high-resolution benthic stable oxygen isotope record of the eastern Mediterranean

Benthic oxygen isotope records of deep marine sedimentary archives have yielded a wealth of information regarding ice sheet dynamics and climate change during the Pleistocene. However, since they often lack independent age control, these records are generally bound by a fixed phase relationship between orbital forcing and the climate response, e.g. ice volume changes. We present the first long (∼1.2 Ma) benthic oxygen isotope record from the eastern Mediterranean, based on ODP Sites 967 and 968, which clearly reflects the behavior of global climate on a glacial–interglacial scale throughout the late Pleistocene time period. The age model for our record is based on tuning the elemental ratio of titanium versus aluminum (Ti/Al) against insolation. The Ti/Al record is dominated by the precession-related changes in northern African climate, i.e. monsoonal forcing, and hence largely independent of glacial–interglacial variability. We found the largest offset between our chronology and that of the widely applied, open ocean stacked record LR04 (Lisiecki and Raymo, 2005) for TVII (∼624 ka), which occurred ∼9 kyr earlier according to our estimates, though in agreement with the AICC2012 δDice chronology of EPICA Dome C (Bazin et al., 2013). Spectral cross-correlation analysis between our benthic δ18O record and 65°N summer insolation reveals significant amounts of power in the obliquity and precession range, with an average lag of 5.5 ± 0.8 kyr for obliquity, and 6.0 ± 1.0 kyr for precession. In addition, our results show that the obliquity-related time lag was smaller (3.0 ± 3.3 kyr) prior to ∼900 ka than after (5.7 ± 1.1 kyr), suggesting that on average the glacial response time to obliquity forcing increased during the mid-Pleistocene transition, much later than assumed by Lisiecki and Raymo (2005). Finally, we found that almost all glacial terminations have a consistent phase relationship of ∼45 ± 45° with respect to the precession and obliquity-driven increases in 65°N summer insolation, consistent with the general consensus that both obliquity and precession are important for deglaciation during the Late Pleistocene. Exceptions are glacial terminations TIIIb, T36 and potentially T32 (and TVII T24 and T34), which show this consistent phase relationship only with precession (only with obliquity). Our findings point towards an early (>1200 ka) onset of the Mid Pleistocene Transition. Vice versa, the timing of TVII, which can only be explained as a response to obliquity forcing, indicates that the transition lasted until at least after MIS 15.

Interglacial occurrence of cold-water corals off Cape Lookout (NW Atlantic): First evidence of the Gulf Stream influence

Climatic and oceanographic changes, as occurring at a glacial–interglacial scale, may alter the environmental conditions needed for the development of prolific cold-water coral reefs and mounds. Studies constraining the temporal distribution of cold-water corals in the NE Atlantic suggested the cyclic changes of the Atlantic Meridional Overturning Circulation as the main driver for the development and dispersal of cold-water coral ecosystems. However, conclusions were hindered by lack of data from the NW Atlantic. Aiming to overcome this lack of data, the temporal occurrence of cold-water corals in the Cape Lookout area along the southeastern US margin was explored by U-series dating. Furthermore, the local influence of the regional water masses, namely the Gulf Stream, on cold-water coral proliferation and occurrence since the Last Glacial Maximum was examined. Results suggest that the occurrence of cold-water corals in the Cape Lookout area is restricted to interglacial periods, with corals being present during the last ~7kyr and also during the Eemian (~125ka). The reconstructed local environmental conditions suggest an offshore displacement of the Gulf Stream and increased influence from the Mid-Atlantic Bight shelf waters during the last glacial period. During the deglacial sea level rise, the Gulf Stream moved coastward providing present-day-like conditions to the surface waters. Nevertheless, present-day conditions at the ocean sea floor were not established before 7.5calka BP once the ultimate demise of the Laurentide ice-sheet caused the final sea level rise and the displacement of the Gulf Stream to its present location. Occasional presence of the Gulf Stream over the site during the Mid- to Late Holocene coincides with enhanced bottom current strength and a slightly higher bottom water temperature, which are environmental conditions that are favorable for cold-water coral growth.

Peatland evolution and associated environmental changes in central China over the past 40,000 years

Central China has experienced stronger summer monsoon during warm periods such as Marine Isotope Stages (MIS) 1 and 3, and weaker summer monsoon during cool periods such as MIS 2. The evolution history of Dajiuhu subalpine peatland in central China can help investigate how the expansion and shrinkage of peatland were associated with monsoonal strength over the last glacial–interglacial cycle. Here we apply bulk organic carbon and molecular biomarkers (hopane and n-alkane) to reconstruct the evolution history for the Dajiuhu peatland over the past 40,000yr. The results indicate fluctuations between lacustrine and peat-like deposition during MIS 3, steady lacustrine deposition during MIS 2, and peatland initiation and expansion during MIS 1 in the Dajiuhu peatland. Therefore, at the glacial–interglacial scale, warmer summer and cooler winter conditions in interglacial periods are crucial to trigger peat deposition, whereas reduced evaporation in glacial period instead of decreased monsoonal-driven precipitation would have played a predominant role in the regional effective moisture balance. However, within the Holocene (MIS 1), monsoonal precipitation changes appear to be the main controller on millennial-scale variations of water-table level of the Dajiuhu peatland.

Paleoenvironmental and climatostratigraphic insights for Marine Isotope Stage 19 (Pleistocene) at the Montalbano Jonico succession, South Italy

Quantitative analyses on pollen and ostracods were performed on the Montalbano Jonico succession (MJS) through Marine Isotope Stages (MIS) 18–21 in order to acquire details of paleoenvironmental changes and climate pattern during MIS 19, which includes the Matuyama–Brunhes paleomagnetic boundary (MBB). While the MJS does not record the MBB, which is the main criterion for defining the Early–Middle Pleistocene boundary, it is a candidate Global Boundary Stratotype Section and Point (GSSP) for the Ionian Stage and Middle Pleistocene Subseries because of its excellent exposure, continuous deposition, and astronomically tuned record between MIS 37 and MIS 16. The new 40Ar/39Ar age of 773.9 ± 1.3ka for volcaniclastic layer V4 allows the age-model for the succession to be refined. Ostracod assemblages show significant paleodepth fluctuations at the glacial–interglacial scale, in agreement with the pollen distality index (Pinus vs. Caryophyllaceae, Amaranthaceae and Ephedra) which also suggests higher values during MIS 19. Pollen analysis provides supplementary paleoclimate data that are valuable to reconstruct temperature variation during the identified phases. A major expansion of steppic vegetation at 790ka in the uppermost MIS 20 correlates with a significant cold and arid Northern Hemisphere climate phase, as documented by a concurrent peak both in North Atlantic ice rafted debris and Mediterranean dust records. Climate conditions recorded in MIS 19.3 were similar to those of the present interglacial, whereas a dry climate phase marks MIS 19.2 at 771.84ka, slightly above the V4 layer. Millennial scale climate variability occurred during MIS 19 as evidenced by two abrupt short-term moderately dry pollen events at 783.5ka and 774.8ka that could be related to a period of reduced geomagnetic field intensity through the MBB transition. Additional environmental/chronological events accompanied this stratigraphic interval: i) the volcaniclastic layers V3 and V4, radiometrically dated at 801.2 ± 19.5ka and 773.9 ± 1.3ka, respectively; ii) the co-occurrence of maximum flooding and thermal maximum in agreement with the previously recorded co-occurrence of the Neopycnodonte community and tropical–subtropical mesopelagic teleostean Bonapartia pedaliota, close to the increase of Quercus; and iii) the maximum depth (MD) between 777.3 and 773.25ka as recorded by outer shelf/upper slope macroinvertebrate fauna, close to the highest value of pollen distality index in MIS 19. Results enhance the understanding of orbital-suborbital paleoenvironmental change during the mid-Pleistocene, and particularly of the climate pattern during MIS 19 and across the MBB.

A voyage through scales, a missing quadrillion and why the climate is not what you expect

Using modern climate data and paleodata, we voyage through 17 orders of magnitude in scale explicitly displaying the astounding temporal variability of the atmosphere from fractions of a second to hundreds of millions of years. By combining real space (Haar fluctuation) and Fourier space analysis, we produce composites quantifying the variability. These show that the classical “mental picture” in which quasi periodic processes are taken as the fundamental signals embedded in a spectral continuum of background “noise” is an iconic relic of a nearly 40 year old “educated guess” in which the flatness of the continuum was exaggerated by a factor of ≈1015. Using modern data we show that a more realistic picture is the exact opposite: the quasiperiodic processes are small background perturbations to spectrally continuous wide range scaling foreground processes. We identify five of these: weather, macroweather, climate, macroclimate and megaclimate, with rough transition scales of 10 days, 50 years, 80 kyrs, 0.5 Myr, and we quantify each with scaling exponents. We show that as we move from one regime to the next, that the fluctuation exponent (H) alternates in sign so that fluctuations change sign between growing (H > 0) and diminishing (H < 0) with scale. For example, mean temperature fluctuations increase up to about 5 K at 10 days (the lifetime of planetary structures), then decrease to about 0.2 K at 50 years, and then increase again to about 5 K at glacial-interglacial scales. The pattern then repeats with a minimum RMS fluctuation of 1–2 K at ≈0.5 Myr increasing to ≈20 K at 500 Myrs. We show how this can be understood with the help of the new, pedagogical “H model”. Both deterministic General Circulation Models (GCM’s) with fixed forcings (“control runs”) and stochastic turbulence-based models reproduce weather and macroweather, but not the climate; for this we require “climate forcings” and/or new slow climate processes. Averaging macroweather over periods increasing to ≈30–50 yrs yields apparently converging values: macroweather is “what you expect”. Macroweather averages over ≈30–50 yrs have the lowest variability, they yield well defined climate states and justify the otherwise ad hoc “climate normal” period. However, moving to longer periods, these states increasingly fluctuate: just as with the weather, the climate changes in an apparently unstable manner; the climate is not what you expect. Moving to time scales beyond 100 kyrs, to the macroclimate regime, we find that averaging the varying climate increasingly converges, but ultimately—at scales beyond ≈0.5 Myr in the megaclimate, we discover that the apparent point of convergence itself starts to “wander”, presumably representing shifts from one climate to another.

VEGETATION COVERAGE AND MONSOON VARIATION RECORDED BY STABLE CARBON ISOTOPE OF LOES SINCE 2.5MA

The organic carbon isotopes of the Luochuan section show that the δ13C values vary from-18.7‰ to-23.5‰,and the downcore distribution of organic carbon isotope could be divided into three stages with similar trends with the magnetic susceptibility in the glacial-interglacial scales.The results demonstrate that 1) the vegetation coverage in the Luochuan area was mainly a mixture of C3 and C4 grassland,2) the δ13C value of loess sequence better recorded the summer monsoon change in the glacial-interglacial scales just as magnetic susceptibility trend.Comparing with the organic carbon isotope of the Duanjiapo and Weinan sections,two times of evident transition on tectonic time scales are recognized since 2.5 Ma.

Coccolithophore carbonate during the last 450 ka in the NW Pacific Ocean (ODP site 1209B, Shatsky Rise)

ABSTRACTThis study focuses on Pleistocene–Holocene sediments from the Shatsky Rise (Ocean Drilling Program Site 1209B, NW Pacific Ocean). We quantify the contribution of calcite made by coccoliths in oceanic sediments, investigating the role of calcareous nannoplankton during the last 450 ka. Coccolith carbonate constitutes 60–90% of bulk carbonate. Coccolith carbonate accumulation rates (CARs) and CaCO3 fine fraction confirm that coccolithophores are major contributors to the carbonate export and accumulation. Primary productivity shows highest values from marine isotope stage (MIS) 12 to 8. Thereafter, although coccolith calcite content remains high, other sediment components, such as as foraminifera and biogenic opal, seem to be favored, perhaps related to an increase in fertilization by eolian dust. Our results demonstrate the important role of coccolithophore production and sedimentation on the regulation of ocean carbonate chemistry on time scales &gt;1000–100 000 years. On glacial–interglacial scales, coccolithophore productivity could have affected deep‐water saturation by buffering deep‐sea CO2 through increased carbonate dissolution episodes. Spectral and wavelet analyses are consistent with CARs primarily driven by glacial–interglacial variability and obliquity‐controlled changes. Coccolith‐based paleoceanographic reconstructions allow us to establish that during the last 450 ka the mid‐latitudes of the NW Pacific are controlled by the dynamics of the El Niño Southern Oscillation perturbations and Boreal Monsoon system.

Calcareous plankton response to orbital and millennial-scale climate changes across the Middle Pleistocene in the western Mediterranean

The paleoenvironmental conditions through MIS 15-9 at the Mediterranean Ocean Drilling Program (ODP) Site 975 were interpreted by high resolution study of calcareous plankton assemblages compared with available δ18O and δ13C records and high resolution paleoclimate proxies from the Atlantic Ocean. Sea Surface Temperatures (SSTs) have been estimated from planktonic foraminiferal assemblages using the artificial neural networks method. Calcareous plankton varied dominantly on a glacial–interglacial scale as testified by the SST record, foraminiferal diversity, total coccolith abundance and changes in warm-water calcareous nannofossil taxa. A general increase in foraminiferal diversity and of total coccolith abundance is observed during interglacials. Warmest SSTs are reached during MIS 11, while MIS 12 and MIS 10 represent the coldest intervals of the studied record. During MIS 12, one of the most extreme glacials of the last million years, occurrence of Globorotalia inflata and of neogloboquadrinids indicates a shoaling of the interface between Atlantic inflowing and Mediterranean outflowing waters. Among calcareous nannofossils the distribution of Gephyrocapsa margereli–Gephyrocapsa muellerae>4μm also supports a reduced Atlantic–Mediterranean exchange during MIS 12. Superimposed on glacial–interglacial variability, six short-term coolings are recognized during MIS 12 and 10, which appear comparable in their distribution and amplitude to the Heinrich-type events documented in the Atlantic Ocean in the same interval. During these H-type events, Neogloboquadrina pachyderma (s) and G. margereli–G. muellerae>4μm increase as a response to the enhanced inflow of cold Atlantic water into the Mediterranean via the Strait of Gibraltar. Mediterranean surface water hydrography appears to have been most severely affected at Termination V during the H-type event Ht4, possibly as a response to a large volume of Atlantic meltwater inflow via the Strait of Gibraltar and/or to freshwater/terrigenous input deriving from local mountain glaciers. Three additional SST coolings are recorded through MIS 14–16, but these are not well correlated with Heinrich-type events documented in the Atlantic Ocean in the same interval; during these cooling episodes only the subpolar Turborotalita quinqueloba increases. These results highlight the sensitive response of the Mediterranean basin to millennial-scale climate variations related to Northern Hemisphere ice-sheet instability and support the hypothesis that the tight connection between high latitude climate dynamics and Mediterranean sea surface water features can be traced through the Middle Pleistocene.

Centennial- to millennial-scale climate shifts in continental interior Asia repeated between warm–dry and cool–wet conditions during the last three interglacial states: evidence from uranium and biogenic silica in the sediment of Lake Baikal, southeast Siberia

We observed timescale-dependent changes in the correlations between biogenic silica (bioSi) and uranium concentrations of bulk sediment from the Buguldeika Saddle, Lake Baikal. There was a positive correlation for the glacial–interglacial (100 kyr) scales over the 180 kyr, with inverse correlations for centennial- to-millennial scales during the last three interglacial states (MIS 5, 3, and 1). The distinction of these correlations between the time scales suggests that the sedimentation processes of bioSi and U differ from each other. The Baikal bioSi concentration record is generally regarded as a paleotemperature proxy, reflecting the diatom production of the lake. On the other hand, we concluded that the temporal variation of the U concentration in the sediment reflected the weathering intensity in the south of Lake Baikal watershed associated with changes in the rainfall/moisture levels.Based on the paleoclimate proxies from Lake Baikal, climates in continental interior Asia are identified as having dry and wet conditions during the glacial and interglacial periods, respectively. This accounts for the rhythms in growth and retreat of ice sheets in Eurasia on glacial–interglacial scales. In the interglacial periods (MIS 5, 3, and 1), the continental interior is, contrary to the glacial–interglacial climate changes, characterized by alternating warm–dry and cool–wet climates with periods in tens of thousands years. In particular, such shifts in the climate stand out prominently at the beginning of MIS 5d. The detrended U record for the last 5.2 kyr shows wet events at 4.3–3.7, 3.2–2.3, 1.8–1.2, and 0.8–0.3 kyr. These events coincide with the timing of the North Atlantic ice-rafted debris events, which were possibly related to solar activity. All this evidence suggests that climate in the continental interior followed significantly different fluctuation patterns in the glacial–interglacial and shorter time scales.

Shoaling of the western equatorial Pacific thermocline during the last glacial maximum inferred from multispecies temperature reconstruction of planktonic foraminifera

Glacial–interglacial scale climate variability in the tropical Pacific is often discussed with reference to modern El Niño-Southern Oscillation (ENSO). However, proxy records show diverse results and result in inconsistent interpretations. To investigate changes in the thermocline depth of the western Pacific warm pool (WPWP), which is strongly related to the strength of the Walker circulation, this study conducted δ18O and Mg/Ca analyses of multispecies planktonic foraminifera for the last 25,000years. Habitat depths and calcification temperatures of seven foraminiferal species were estimated by comparing measured δ18O values with the predicted values by assuming isotopic equilibrium. Reconstructed Mg/Ca-based temperatures, calculated with a newly calibrated multispecies equation, showed ~1–2°C larger cooling at the thermocline depth than that at the surface during the last glacial maximum (LGM), suggesting shoaling of the thermocline. Furthermore, fresher surface water in the WPWP, which is attributed to the suppressed evaporation by low sea surface temperature, suggests the weakened deep atmospheric convection. These results suggest that the Walker circulation during the LGM was weaker than that during the late Holocene.

Driving forces of mid-Holocene vegetation shifts on the upper Tibetan Plateau, with emphasis on changes in atmospheric CO 2 concentrations

Numerous pollen records across the upper Tibetan Plateau indicate that in the early part of the mid-Holocene, Kobresia-rich high-alpine meadows invaded areas formerly dominated by alpine steppe vegetation rich in Artemisia. We examine climate, land-use, and CO 2 concentration changes as potential drivers for this marked vegetation change. The climatic implications of these vegetational shifts are explored by applying a newly developed pollen-based moisture-balance transfer-function to fossil pollen spectra from Koucha Lake on the north-eastern Tibetan Plateau (34.0°N; 97.2°E; 4540 m a.s.l.) and Xuguo Lake on the central Tibetan Plateau (31.97°N; 90.3°E; 4595 m a.s.l.), both located in the meadow-steppe transition zone. Reconstructed moisture-balances were markedly reduced (by ∼150–180 mm) during the early mid-Holocene compared to the late-Holocene. These findings contradict most other records from the Indian monsoonal realm and also most non-pollen records from the Tibetan Plateau that indicate a rather wet early- and mid-Holocene. The extent and timing of anthropogenic land-use involving grazing by large herbivores on the upper Tibetan Plateau and its possible impacts on high-alpine vegetation are still mostly unknown due to the lack of relevant archaeological evidence. Arguments against a mainly anthropogenic origin of Kobresia high-alpine meadows are the discovery of the widespread expansion of obviously ‘natural’ Kobresia meadows on the south-eastern Tibetan Plateau during the Lateglacial period indicating the natural origin of this vegetation type and the lack of any concurrence between modern human-driven vegetation shifts and the mid-Holocene compositional changes. Vegetation types are known to respond to atmospheric CO 2 concentration changes, at least on glacial–interglacial scales. This assumption is confirmed by our sensitivity study where we model Tibetan vegetation at different CO 2 concentrations of 375 (present-day), 260 (early Holocene), and 650 ppm (future scenario) using the BIOME4 global vegetation model. Previous experimental studies confirm that vegetation growing on dry and high sites is particularly sensitive to CO 2 changes. Here we propose that the replacement of drought-resistant alpine steppes (that are well adapted to low CO 2 concentrations) by mesic Kobresia meadows can, at least, be partly interpreted as a response to the increase of CO 2 concentration since 7000 years ago due to fertilization and water-saving effects. Our hypothesis is corroborated by former CO 2 fertilization experiments performed on various dry grasslands and by the strong recent expansion of high-alpine meadows documented by remote sensing studies in response to recent CO 2 increases.

Quaternary river terraces in England: Forms, sediments and processes

Flights of Quaternary river terraces in south and east England have common characteristics involving low-gradient planed or irregular bedrock surfaces and single or multi-storey gravel deposits. Rather than depending on warm–cold or cold–warm transitions, it is suggested that bedrock planation, “working depths” of gravel and later-stage (relatively shallow) aggradations are all dominantly of cold-climate origin. Basal sediments show active incorporation of plucked and periglacially-shattered materials, whilst super-incumbent units incorporating up-catchment and slope-derived materials demonstrate later cold-stage sediment influx and consequent cessation of active bedrock erosion. Channel activity effecting both planation and deposition are reviewed, together with the detailed sedimentology of gravelly sediments which show evidence of both autogenic processes (bar migration, channel switching and infilling, and truncation of upper sedimentation units), cold-climate indicators (turbation, ice-wedge casts, and frozen block transport), and (specifically for the last glacial–interglacial cycle) varying sediment flux as climates changed. Both interglacial and “transitional” activities are believed to be of lesser morphological significance, whilst prior uplift is taken as enabling rather than being a generator of terrace within the timescale of a glacial–interglacial cycle. Variations within cold-stage climates, varying sediment influx and channel-belt bedrock erosion are stressed as dominating mid-catchment and mid-latitude Quaternary terracing at the glacial–interglacial scale.

A test of stable carbon isotope record characteristics of phreatic and outcrop profiles

Current research on stable carbon isotopes in Chinese loess mostly focuses on the natural outcrops located on the sides of gullies formed by rainfall erosion on the Loess Plateau. In those environments, late processing may influence the carbon isotopic composition of loess-paleosol. This issue is not widely reported in the literature. A comparative analysis of carbon isotopes in phreatic and outcrop profiles was performed for two sites located on the Chinese Loess Plateau (CLP). The results for the Luochuan and Xifeng sites indicate that the distribution characteristics for carbon isotopes (δ 13C TOC and δ 13C TIC) of phreatic and outcrop profiles present a consistent trend of changes on the glacial–interglacial scales, respectively. High δ 13C TOC values are found in paleosol (S 0 and S 1) and weak paleosol (L 1SS 1), and low values are found in the loess stage; however, the high δ 13C TIC values are found in less pedogenically altered loess layers, and low δ 13C TIC values are found in paleosol horizons. Therefore, late processing does not appear to influence the stable carbon isotope composition of the loess from natural outcrop profiles on glacial–interglacial scales for studied profiles. This finding indicates that the carbon isotope composition of easily excavated natural outcrop profiles can accurately record the characteristics of paleo-vegetation and paleo-climate on glacial–interglacial scales.

A comparative study of boron isotopes and trace elements of the marine foraminifers during the last glacial maximum and Holocene

Boron isotopic compositions in marine planktonic foraminifers can record changes in seawater pH and hence provide a new tool to reconstruct the changes of paleo-atmospheric PCO2. Here a comparative study was done on boron isotopes and trace element abundances of planktonic foraminifers, Globigerinoides sacculifer and Globigerinoides ruber from three tropical ODP sites, Site 806B, Site 664C and Site 999A. It is demonstrated here that G. ruber is a better species to be used for downcore boron isotope analyses on glacial-interglacial scales, particularly since G. sacculifer is prone to dissolution causing unexpectedly light boron isotopes during glacial times. It has been shown that the trace elements composition of foraminifers (i.e., Mg/Ca, U/Ca, B/Ca, and Li/Ca) can reflect the variations of seawater temperature and/or [CO32−]. However, all of these systems are highly species de- pendent and affected by other vital effects, and it is therefore important to perform necessary calibrations when applying them to the downcore analyses.

A 27-kyr record of environmental change in central Asia inferred from the sediment record of Lake Hovsgol, northwest Mongolia

Geochemistry of a sediment core from Lake Hovsgol, northwest Mongolia provides a continuous, 27-kyr history of the response of the lake and the surrounding catchment to climate change. Principle component (PC) analysis of 19 major and trace elements, total inorganic carbon (TIC), and total organic carbon (TOC) in the bulk sediment samples revealed that the 21 chemical components can be grouped into four assemblages—group-1: Na, Mg, Ca, Sr, and TIC, hosted in carbonate minerals (calcite, dolomite, and magnesian calcite); group-2: Ni, Cu, and Zn, recognized as biophilic trace metals, and TOC; group-3: Al, K, Ti, V, Fe, Rb, Cs, Ba, and Pb, composed of rock-forming minerals; and group-4: Cr, Mn, and As, sensitive to the redox condition of the sediment. The four element assemblages originated from three relevant processes. Group-1 and group-2 components are authigenic products and comprise the end member on the PC-1 score, whose variation reflects changes in the water volume, i.e. the balance between precipitation and evaporation (P/E). Group-3 components from detrital materials of the catchment contribute to the PC-2 score, whose variability indicates erosion/weathering intensity in the drainage basin, which might be controlled by the amount of vegetation cover associated with moisture change. The group-4 components of redox-sensitive elements contribute to the PC-3 score and are not an end member because of their small amount. The first two PC scores suggest a sequential record of paleo-moisture evolution in central Asia. The P/E balance in the Lake Hovsgol region, inferred from the PC-1 score, gradually increased during the glacial/interglacial transition. This resembles climate change of the North Atlantic region on the glacial–interglacial scale, but does not reflect the abrupt climate shifts such as the warm Bolling-Allerod and the cold Younger Dryas of the North Atlantic on the millennial scale. A periodic variation of ~8.7 kyr was observed in the PC-2 score profile of detrital input to Lake Hovsgol over the last glacial and Holocene. The decrease in detrital input coincided with the copious supply of moisture from the Asian monsoon regime and the North Atlantic westerly winds to the Baikal drainage basin, which includes Lake Hovsgol. Our geochemical records from Lake Hovsgol demonstrate that the climate system of interior continental Asia was strongly influenced by change on both Milankovitch and sub-Milankovitch scales.

Biochronology and paleoceanography of late Pleistocene and Holocene calcareous nannofossil abundances across the Arctic Basin

Late Pleistocene and Holocene stratigraphic distributions of calcareous nannofossils have been investigated in seven short cores raised from four major ridges in the Arctic Basin and in one core from the Norwegian Sea. Observed assemblages in the Arctic Ocean cores mostly represent the influence of Atlantic shallow waters. On the Gakkel and Lomonosov Ridges, a distinct cross-over in abundance occurs between Emiliania huxleyi and Gephyrocapsa spp. in the upper part of Marine Isotope Stage (MIS) 3. The lowest abundances of indigenous calcareous nannofossils occur in the southern Mendeleev Ridge core and in the two Alpha Ridge cores. These areas show a minor production of calcareous nannoplankton, probably because of the progressively diminishing influence of shallow Atlantic waters with the increasing distance from the Fram Strait gateway, the key conduit of shallow water inflow to the Arctic Ocean. It remains unknown whether or not the observed abundance patterns have been modified by glacial–interglacial scale changes in preservation. Obvious reworking becomes intensified around MIS boundaries.

Strong asymmetry of hemispheric climates during MIS-13 inferred from correlating China loess and Antarctica ice records

Abstract. We correlate the China loess and Antarctica ice records to address the inter-hemispheric climate link over the past 800 ka. The results show a broad coupling between Asian and Antarctic climates at the glacial-interglacial scale. However, a number of decoupled aspects are revealed, among which marine isotope stage (MIS) 13 exhibits a strong anomaly compared with the other interglacials. It is characterized by unusually positive benthic oxygen (δ18O) and carbon isotope (δ13C) values in the world oceans, cooler Antarctic temperature, lower summer sea surface temperature in the South Atlantic, lower CO2 and CH4 concentrations, but by extremely strong Asian, Indian and African summer monsoons, weakest Asian winter monsoon, and lowest Asian dust and iron fluxes. Pervasive warm conditions were also evidenced by the records from northern high-latitude regions. These consistently indicate a warmer Northern Hemisphere and a cooler Southern Hemisphere, and hence a strong asymmetry of hemispheric climates during MIS-13. Similar anomalies of lesser extents also occurred during MIS-11 and MIS-5e. Thus, MIS-13 provides a case that the Northern Hemisphere experienced a substantial warming under relatively low concentrations of greenhouse gases. It suggests that the global climate system possesses a natural variability that is not predictable from the simple response of northern summer insolation and atmospheric CO2 changes. During MIS-13, both hemispheres responded in different ways leading to anomalous continental, marine and atmospheric conditions at the global scale. The correlations also suggest that the marine δ18O record is not always a reliable indicator of the northern ice-volume changes, and that the asymmetry of hemispheric climates is one of the prominent factors controlling the strength of Asian, Indian and African monsoon circulations, most likely through modulating the position of the inter-tropical convergence zone (ITCZ) and land-sea thermal contrasts.

Processes controlling the geochemical composition of the South China Sea sediments during the last climatic cycle

Sediments of the upper 28.2 m of Ocean Drilling Program (ODP) Site 1145 from the northern South China Sea (SCS) were analyzed for their geochemical composition. Most of the major and trace elements exhibit significant fluctuations at glacial–interglacial scales, implying a close relation with regional and global climate change. Al-normalized elemental ratios can be subdivided into three principal components (PC). PC1 (e.g., Ca/Al, Ba/Al, Sr/Al) displays significant glacial–interglacial variation and is related to paleoproductivity in the northern SCS. PC2 (e.g., K/Al, Mg/Al, Rb/Al) is associated with the degree of chemical weathering in the source regions and shows little glacial–interglacial variation. PC3 (e.g., Ti/Al, Zr/Al) reflects the relative contribution of coarse- and fine-grained materials in the terrigenous components of the SCS sediments, likely associated with changes in sea level and monsoon-induced fluvial input. Spectral analyses indicate that paleoproductivity (i.e., Ba/Al) in the South China Sea lags Hulu/Sanbao speleothem δ 18O record (an indicator of annual average meteoric precipitation) by 102° and Indian summer monsoon (multi-proxy stack) by 23° at the precession band, indicating a close relationship with the Indian summer monsoon. However, the chemical weathering degree in the source area (PC2) is not sensitive to monsoon-related changes at the precession band during the last climatic cycle.

Coccolith evidence for Quaternary nutricline variations in the southern South China Sea

Nutricline variations during the last 1560 ka in the southern South China Sea are reconstructed using the relative abundance of the coccolithophore Florisphaera profunda in ODP Site 1143. Nutricline depth shows both long-term large magnitude variation and high frequency glacial–interglacial variation. On the long-term scale, the nutricline experienced four significant changes, which occurred at 900, 480, 250, and 50 ka respectively. According to these changes, five stages could be recognized during the last 1560 ka. From 1560 to 900 ka, the nutricline depth increased gradually. Around 900 ka, the nutricline abruptly shallowed and then remained stable until 480 ka. The nutricline was shallow during the time interval between 480 and 250 ka. At 250 ka, it deepened again and increased gradually until 50 ka. After 50 ka, the nutricline depth decreased gradually to modern values. On the glacial–interglacial scale, the variations in nutricline depth show different patterns before and after 900 ka. Before 900 ka, the nutricline was deep during glacial periods and shallow during interglacials. However, after 900 ka, the nutricline was deep during interglacials and shallow during glacials. Spectral analysis of the relative abundance of F. profunda shows a similar trend. In addition to the eccentricity (113, 76 ka), obliquity (55, 39 ka), and precession (24, 19 ka), we also find a 431 ka cycle. The former three periods reflect glacial–interglacial variations in nutricline, and the period of 431 ka reflects long-term variations in nutricline. We suggested that the variations in nutricline in the southern South China Sea were caused by global and regional climate changes. Glacial–interglacial variations in nutricline are mainly controlled by the East Asian monsoon, and the long-term variations might be related to the global climatic events, such as the mid-Pleistocene Revolution and the mid-Brunhes event.