Glacial-interglacial Scales Research Articles

High-northern-latitude forcing modulated fluvial‒aeolian interplay during the Last Glacial Period in the Mu Us Desert, northern central China

Substantial stratigraphic evidence from the Late Quaternary at orbital- and millennial‒scales indicates that the formation and evolution of aeolian‒sand landscapes results from the interaction between regional aeolian and fluvial‒lacustrine activity. In many deserts (sandy lands) across the planet, fluvial‒aeolian interplay at the glacial‒interglacial scale since the Late Quaternary has been responsive to monsoonal precipitation, and linked to atmospheric CO2 and periodic variations in insolation. However, the patterns of fluvial‒aeolian interplay and their relationship with climate change are not always consistent over a given timescale during the Late Pleistocene. The paucity of studies on fluvial‒aeolian interplay in the Mu Us Desert (MUD) during the Last Glacial Period (LGP) hinders our understanding of the relationships between the dynamics of aeolian and fluvial‒lacustrine landscapes and their drivers. Here, based on a comprehensive field survey and stratigraphic dating of a typical fluvial‒aeolian sequence in the MUD, we synthesized the chronology of fluvial‒lacustrine and aeolian strata during the LGP. Our results show that extensive development of a fluvial‒lacustrine environment in the MUD during the LG mainly occurred during the late Last Glacial Maximum (LGMa, 20.1–18.4 ka BP) and Bølling/Allerød interstadial (14.8–12.6 ka BP), while thicker aeolian sands extensively accumulated during the Heinrich Stadial 2, Heinrich Stadial 1, and Younger Dryas periods. However, the development of fluvial‒lacustrine environments showed considerable spatial heterogeneity, mainly occurring on the southern and eastern margins of the desert, while they were extremely rare in the central and western parts of the desert owing to strong wind erosion. Comparisons with other high-resolution geological records from the Asian monsoon region reveal that the fluvial‒aeolian interplay in the MUD during the LGP was highly consistent with the evolution pattern of the East Asian summer monsoon (EASM) on the millennial‒centennial scale, and indicate that the development of fluvial‒lacustrine environments was mainly controlled by an increase in monsoonal precipitation during strengthening of the EASM. Consistency with other broad changes at high latitudes in the Northern Hemisphere suggests that fluvial‒aeolian interplay during the LGP in northern Chinese deserts, as represented by the MUD, was mainly controlled by changes in high-northern-latitude ice volume and the Atlantic Meridional Overturning Circulation.

Early diagenesis effects on Mg/Ca thermometry during the MIS 9–5 in the Gulf of Mexico, evaluation on foraminifera tests and geochemical signals

Analyses of Mg/Ca in planktonic foraminifera Globigerinoides ruber (white) tests were made to evaluate burial influences and preservation before reconstructing paleotemperatures. We studied samples from the eastern Gulf of Mexico marine sediment core EN32-18PC at a glacial-interglacial scale. The core chronology is based on planktonic foraminifera biostratigraphy and δ18OG. ruber stratigraphy, showing that the core spans the biozones V2 - Y and the Marine Isotope Stages (MIS) 9 to 4 (i.e., 319.3 to 66.0 ka). Based on Scanning Electron Microscopy (SEM) and LA-ICP-MS analysis of Mg/Ca, Sr/Ca, Mn/Ca, Fe/Ca, and Mn/Fe ratios, we found evidence of test alteration by early diagenesis. Some tests show material infillings, pore reduction, polygonal patterns between pores, and wall thickening by calcareous overgrowth. Values of Mg/Ca >6 mmol/mol and δ18O > 2‰ must be carefully evaluated before reconstructions. Values of Mn/Ca >0.6 mmol/mol and Fe/Ca >0.2 mmol/mol indicate the adherence of clays or coatings (Mn or Fe oxides). Nevertheless, we did not find evidence of severe diagenesis in the tests (like recrystallization or euhedral crystals grown by inorganic precipitates); thus, their alteration degree is incipient, and calcite overgrowth is the most severe diagenetic effect during MIS-6 and early MIS-5. After calculating paleotemperatures, we obtained SST values comparable to other cores in the region (i.e., 24 °C - 31 °C). The paleotemperature record fluctuates over MIS 9 to 5, in agreement with the insolation intensity cycle controlled by precession. However, suborbital trends could not be reconstructed due to the studied effects imprinted on the specimens.

An Intertropical Convergence Zone shift controlled the terrestrial material supply on the Ninetyeast Ridge

Abstract. Among various climate drivers, direct evidence for the Intertropical Convergence Zone (ITCZ) control of sediment supply on the millennial scale is lacking, and the changes in ITCZ migration demonstrated in paleoclimate records need to be better investigated. Here, we use clay minerals and Sr–Nd isotopes obtained from a gravity core on the Ninetyeast Ridge to track the corresponding source variations and analyze the relationship between terrestrial material supply and climatic changes. On the glacial–interglacial scale, chemical weathering weakened during the North Atlantic cold-climate periods and falling sea level hindered the transport of smectite into the study area due to the exposure of Andaman and Nicobar Islands. However, the influence of the South Asian monsoon on the sediment supply was not obvious on the millennial scale. We suggest that the north–south migration of the ITCZ controlled the rainfall in Myanmar and further directly determined the supply of clay minerals on the millennium scale because the transport of smectite was highly connected with the ITCZ location; thus, the regional shift of the ITCZ induced an abnormal increase in the smectite percentage during the late Last Glacial Maximum (LGM) in our records. The smectite percentage in the studied core is similar to distinct ITCZ records but different in some periods, revealing that regional changes in the ITCZ were significantly obvious, the ITCZ is not a simple north–south displacement, and closer connections occurred between the Northern–Southern Hemisphere in the eastern Indian Ocean during the late LGM.

Glacial-interglacial evolution of seasonal cooling events documented by land-snail eggs from Chinese loess

The alternations of glacial and interglacial cycles are a classical feature of Quaternary climatic evolution and have been demonstrated to be closely related to seasonal insolation changes at high northern latitudes. Therefore, seasonal features may provide insights into glacial-interglacial cycles. However, mainly due to the lack of long time series of seasonally sensitive proxies, little is known about seasonal changes on the glacial-interglacial scale. The unhatched eggs preserved in sediments can serve as a proxy of seasonal cooling events (e.g., cold spells) since biological principles indicate that egg hatching is sensitive to temperature changes, and cooling-event-induced low temperatures during the reproductive season are unfavorable for eggs to hatch. Vertebrate eggs are well documented in the geological records, but they rarely provide continuous records through time. Here we present a high-resolution time series of land-snail eggs from the Chinese Loess Plateau, spanning the last three glacial-interglacial cycles. The results show that seasonal cooling events, indicated by peaks in egg abundance, are strong during glacial inceptions and climate cooling shifts of the marine isotope stages (MIS) 7e/7d, MIS 5e/5d, MIS 5c/5b and MIS 3/2. They tend not to occur during deglacials. They may result in low temperatures unfavorable for egg hatching during the reproductive season. Although several factors may be involved, seasonal cooling events in the Chinese Loess Plateau seem to be positively and more closely related to high-northern-latitude ice sheet growth. This finding may provide a new perspective for understanding glacial-interglacial evolution.

Paleoproductivity evolution in the northeastern Indian Ocean since the last glacial maximum: Evidence from biogenic silica variations

Changes of marine paleoproductivity could reflect the history of marine biogenic cycle process, and exploring the evolution and mechanism of marine productivity are important for understanding the global biogenic factor cycle and its roles in past, present and future climate change. Based on the analysis of biogenic silica (BSi) in core ADM-159 and core BoB-56 from the northeastern Indian Ocean, high-resolution curves of paleoproductivity evolution were obtained. The results show that the BSi contents in these two core sediments are less than 1.8%, mainly affected by the low supply of silica skeleton and the high dissolution and dilution of terrigenous materials. Distinct fluctuations in BSi contents could still be reflected, and their relative values were controlled by chemical weathering processes and terrigenous material supply. The power spectrum analysis of BSi content shows reliable periodicities of 5.86 kyr, 1.95 kyr, 1.35 kyr and 1.09 kyr, suggesting that millennial-scale BSi value might be affected by solar activity. Based on the BSi mass accumulation rates (MAR), lower paleoproductivity during the glacial period and higher level during the Holocene were revealed in our records, which matched well with the global climate changes. Temperatures controlled by low latitude insolation changes could influence the nutrient substance supply, further promote productivity variations at the glacial–interglacial scale. Some millennial events, such as the Heinrich stadial 1 (HS1) and Younger Dryas (YD) periods, showed relatively low paleoproductivity, while the Bølling/Allerød (B/A) period and the middle Holocene showed higher paleoproductivity. These millennial paleoproductivity fluctuations may be due to the changes of nutrient supply caused by Indian summer monsoon (ISM). In addition, the BSi MAR of core ADM-159 and core BoB-56 are well correlated with the isotopic compositions of Greenland ice core and Chinese speleothem, providing robust evidence for the link between northeastern Indian Ocean paleoproductivity and global climate change.

Nutrient utilization and diatom productivity changes in the low-latitude south-eastern Atlantic over the past 70 ka: response to Southern Ocean leakage

Abstract. Eastern boundary upwellings (EBUs) are some of the key loci of biogenic silica (opal) burial in the modern ocean, representing important productive coastal systems that extraordinarily contribute to marine organic carbon fixation. The Benguela upwelling system (BUS), in the low-latitude south-eastern Atlantic, is one of the major EBUs and is under the direct influence of nutrient-rich Southern Ocean waters. Quantification of past changes in diatom productivity through time, in response to late Quaternary climatic change, feeds into our understanding of the sensitivity of EBUs to future climatic perturbations. Existing sediment archives of silica cycling include opal burial fluxes, diatom assemblages, and opaline silicon isotopic variations (denoted by δ30Si). Burial fluxes and siliceous assemblages are limited to recording the remains reaching the sediment (i.e. export), and δ30Si variations are complicated by species-specific influences and seasonality. Here, we present the first combined δ30Si record of two large centric diatoms from the BUS, encompassing full glacial conditions to the Holocene. In addition to export, our new data allow us to reconstruct the utilization of dissolved Si in surface waters in an area with strong input from Southern Ocean waters. Our new archives show that there was enhanced upwelling of Southern Ocean Si-rich water accompanied by strong silicic acid utilization by coastal dwelling diatoms during Marine Isotope Stage 3 (MIS3; 60–40 ka). This pulse of strong silicic acid utilization was followed by a weakening of upwelling and coastal diatom Si utilization into MIS2, before an increase in pelagic diatom Si utilization across the deglaciation. We combine our findings with mass balance model experiments to show that changes in surface water silica cycling through time are a function of both upwelling intensity and utilization changes, illustrating the sensitivity of EBUs to climatic change on glacial–interglacial scales.

Post-IR Irsl Chronology of Paleo-Lacustrine Sediments from Yardangs in the Qaidam Basin, NE Tibetan Plateau

The Qaidam Basin preserves the largest Yardang field on Earth, and yardangs are intriguing landforms for studies of the paleo-environment and aeolian processes. Formation of yardangs involved both the initial lacustrine deposition and the subsequent wind-erosion processes. However, the timings of both processes in the Qaidam Basin are still controversial due to limited age data and unsuitable dating methodology. In this paper, we first compared two optical dating methods to determine the suitable one for the study area, then investigated the geomorphic processes based on the new ages. Two-step post-IR IRSL (pIRIR) and multi-elevated-temperature pIRIR (MET-pIRIR) methods of feldspar, were applied to date lacustrine sediments on the top parts of yardangs to decipher the transition time from depositional to an erosional environment. Comparisons of the two methods demonstrated that the influence from anomalous fading was very minimal thus negligible for MET-pIRIR method, as proved by the De plateau between MET-pIRIR<sub>250</sub> and MET-pIRIR<sub>290</sub>; while the pIR50IR<sub>250</sub> signals suffered from fading obviously, which was difficult to be corrected due to the high De close to saturation. Consequently, the chronology in this study was based on the MET-pIRIR<sub>250</sub> method, potentially offering reliable ages of over 200 ka. Seven MET-pIRIR<sub>250</sub> ages of 201–336 ka suggested that a mega-Qaidam Lake (>2714 m a.s.l. on Google Earth) maintained until Marine Isotopic Stage (MIS) 7. The absence of sediments since <i>ca.</i> 200 ka implied wind-erosion and yardang formation since MIS6. This transition from lacustrine to a wind-erosion environment was interpreted as a response to the glacial-interglacial scale climatic changes.

Variability and environmental significance of organic carbon isotopes in Ganzi loess since the last interglacial on the eastern Tibetan Plateau

The stable carbon isotope composition of total organic matter (δ13Corg) in loess deposits has been widely used as a valuable proxy for reconstructing terrestrial paleovegetation. Aeolian loess is widely distributed on the high Tibetan Plateau (TP); however, little is known about the variability and paleoenvironmental implications of the δ13Corg of Tibetan loess. We conducted detailed δ13Corg and total organic carbon analyses of the Ganzi loess sequence on the eastern TP since the last interglacial (LIG). Our results indicate that the δ13Corg values of loess during glacial periods were lower than those during interglacial periods, with more negative values associated with the cold stages since the LIG on the eastern TP. The reconstruction of vegetation from the δ13Corg data indicated that C3/C4 mixed vegetation was dominated by C3 vegetation, and that the relative abundance of C4 vegetation during the glacial stages was much lower than that during MIS 5 and MIS 3. The high relative abundance of C4 plants during MIS 5 and MIS 3 suggested an increased proportion of C4 plants during the warm period. Since the LIG, the change in the abundance of C3/C4 vegetation in the Ganzi region has been mainly controlled by climate change at the glacial–interglacial scale. The combined effect of temperature, precipitation, and atmospheric CO2 caused changes in the paleovegetation on the eastern TP.

Provenance of terrigenous sediments in the central Bay of Bengal and its relationship to climate changes since 25\u2009ka

Lithogenic grain-size, geochemical elements of core sediments from the central Bay of Bengal (BoB) were analyzed to identify sediment provenance and explore the regional “source-sink” processes since 25 ka. Based on the geochemical compositions of core BoB-88, relative contributions of three end-member sources (Himalayan, Myanmar, Indian Peninsula) were calculated and support the general understanding that Himalayan sources were dominant since the last glacial period, which could reach 70% on average in our study. However, sediments from the Indian Peninsula and Myanmar also contributed nonnegligible materials to the central BoB since 25 ka, especially the former shows an obvious increase since 7.5 ka, which was neglected in the previous studies. To trace the sedimentary processes of different provenance terrigenous materials and reveal their response to climate changes, four typical stages with obvious provenance percent variation, including the Last Glacial Maximum period (LGM), Heinrich 1 (H1), Early Holocene Climate Optimum (EHCO), and mid-late Holocene period, were highlighted and discussed. Sea-level change played a dominant role in the glacial-interglacial scale by controlling the transition of deposition center between the shelf/subaquatic delta and the Bengal Fan, especially for sediment from the Himalayan source, exerting influence on contributions from different provenance end-members between the Holocene and the last glacial period. The opposite directions of the southwest and northeast monsoon current in the BoB have a significant influence on the transfer of the Indian Peninsula and Myanmar materials to the studied core, which led to an increase of up to 11% than the average of the Indian Peninsula and a decrease of up to 9% than the average of the Myanmar source during the Holocene. The balance between sea level (limited transport to the deep sea) and ISM (improved erosion production and output to the BoB) during the mid-late Holocene period maintained the stability of Himalayan source percent. Due to the control of the ISM variations, the Himalayan source showed a decrease of up to 20% and 21% than average during the LGM and H1 period, respectively, and an increase of up to 14% than average during the EHCO. The Myanmar source exhibited fluctuations around the average value during the LGM period, but an obvious increase of up to 21% during the H1 period and a decrease up to 8% during the EHCO, while the Indian Peninsula source percent showed little variations during the LGM and H1, but a decrease of up to 6% during the EHCO. Obvious 1/4 processional and millennial periodic changes (5.0 kyr and 1.1 kyr) of contribution proportions from three end-members provide additional evidences for the climate control on the regional source-sink processes and proved the controlled mechanisms of the orbital parameters and possible influence from the solar activity on millennial scales.

Temporal–spatial variations in aeolian flux on the Chinese Loess Plateau during the last 150 ka

AbstractAeolian dust deposits from continent and ocean have been extensively investigated to reflect past changes in source aridity and atmospheric circulations. Aeolian flux (AF) as a quantitative dust proxy has been widely used in both palaeoenvironmental reconstruction and numerical simulation. However, available AF data on the Chinese Loess Plateau (CLP) is too limited to assess the temporal–spatial variations at glacial–interglacial timescales, and therefore cannot be used as robust input parameters in palaeoclimate models. Here we investigate eight loess profiles along two N–S-aligned transects on the CLP to quantitatively estimate the AF variations over the last glacial–interglacial cycle. We first establish a refined chronological framework based on optically stimulated luminescence chronology and pedostratigraphic correlation. AF was then estimated by multiplying the sedimentation rate and bulk density. The results show that the AF increases from 2–18 g cm−2 ka−1 in the southeastern CLP to 14–105 g cm−2 ka−1 in the northwestern CLP. At glacial–interglacial scales, the AF varies from 2–20 g cm−2 ka−1 during the last interglacial to 8–105 g cm−2 ka−1 in the last glaciation. Due to more spatial coverage and better age constraints, our AF data can be used to refine other AF datasets and to improve the proxy–model comparison.

East Asian winter monsoon evolution since the late Pliocene based on a pollen record from Lake Xingkai, northeast Asia

AbstractA 328.58 m drill core (XK12) was recovered from lacustrine–alluvial sediments in the Xingkai Basin, northeast China, with the aim of obtaining a high-resolution pollen record of East Asian winter monsoon (EAWM) evolution since 3.6 Ma. An index based on the pollen record of thermophilous trees and terrestrial herbs is used as an indicator of winter temperature conditions controlled by the EAWM, at the glacial–interglacial scale. Primary age control was established based on lithostratigraphy and magnetostratigraphy, and then the pollen index was correlated to the LR04 global benthic δ18O record and finally tuned to Earth orbital obliquity to produce a high-resolution astronomical time scale. The pollen record indicates that the EAWM underwent two stepwise enhancements at 2.8 and 1.6 Ma. These events are consistent with paleoclimatic records of mean quartz grain size from the Chinese Loess Plateau, and they are also in accord with the initiation and intensification of Northern Hemisphere glaciation. Our findings suggest that the variability of the EAWM since 3.6 Ma was primarily controlled by changes in global ice volume and climatic cooling.

Sedimentary responses to the sea level and Indian summer monsoon changes in the central Bay of Bengal since 40 ka

We present a sedimentary multi-index comprehensive study on sediment core BoB-56 collected from the central Bay of Bengal (BoB) with an attempt to understand the sedimentary processes and their responses to sea level change and the Indian summer monsoon (ISM) since the last glacial period. We revealed the temporal evolution of terrigenous input and provided a brief discussion of the implications of our sedimentary indexes for ocean productivity. Then, we proposed a simple depositional model in the central BoB since the last glacial period. As indicated by the geochemistry, mineralogy and physical indexes, including linear sedimentary rate (LSR), Ti/Ca, Rb/Sr and quartz/calcite ratios, the terrigenous input was higher during the last glacial period than during the Holocene period, and the sea level is suggested to be responsible for this significant distinction through the depositional center transition in the BoB at the glacial-interglacial scale. Similar terrigenous input patterns with solar radiation and precipitation before 18 ka indicated that the monsoon controlled these situations at precession-related scales. Synchronous changes with records in the north Atlantic Ocean during the last deglaciation and early Holocene suggested terrigenous input responses to the climate changes that also recorded by high latitude sedimentary system at millennial scales. Indicated by the element (CaCO3, Ba and Sr) authigenic accumulation rates, the paleoproductivity in the central BoB was at a roughly equivalent level during the last glacial period and the Holocene period. Different terrestrial nutrient inputs and ISM-related ocean surface stratifications were suggested to be responsible for the level of paleoproductivity. This study provides a brief understanding of the sedimentary response to the climate and emphasizes the different roles of the sea level and ISM in the central BoB.

Total organic carbon sensitivity to climate change from Lake Qinghai sediments at different time scales

湖泊沉积物总有机碳（TOC）含量通常作为表征流域和湖泊生产力的指标，在亚洲季风区也常常被当作夏季风的代用指标，被广泛应用于气候与环境变化研究.本文梳理了过去千年、全新世以及冰期-间冰期时间尺度上青海湖沉积物TOC的变化特征，并探讨了其指示气候变化的敏感性与有效性.结果表明，过去千年青海湖沉积物TOC含量与区域暖季温度和降水表现出较为一致的周期性波动.通过对比全新世区域夏季温度、基于孢粉的降水定量重建结果，以及湖泊水位、风沙活动反映的湿度状况等，发现不能简单地将青海湖沉积物TOC含量或沉积通量作为夏季风强度或者季风降水强度的代用指标.青海湖沉积物TOC含量在冰期和间冰期表现出巨大的差异，指示了冰期-间冰期时间尺度上较大的温度与降水变幅.因此，不同地域条件及不同时间尺度下，湖泊沉积物TOC对气候变化的敏感性不同，将湖泊沉积物TOC含量作为亚洲夏季风的代用指标需要特别谨慎，特别是在高寒气候区.;Total organic carbon (TOC) content of lacustrine sediments has been widely used in the climatic and environmental studies as a proxy of productivity in the catchment and lake water; it is also used as an index of monsoon precipitation in the regions dominated by the Asian summer monsoon. In the present paper, we summarized the variation characteristics of TOC in Lake Qinghai sediments at scales of past millennium, the Holocene and the glacial-interglacial; and discussed the sensitivity and applicability of TOC for indicating climate change. The results indicate that the variation of TOC in Lake Qinghai sediments during the past millennium is consistent with that of regional warm season temperature, as well as the precipitation. After compared with regional summer temperature, pollen-based precipitation reconstruction results, and moisture evolution inferred from water level of lakes and aeolian activity history, we found that TOC content or flux cannot be regarded as a reliable index of the intensity of summer monsoon or monsoonal precipitation during the Holocene. TOC content from Lake Qinghai during the Holocene and the last glacial were distinctly different, indicating huge difference of precipitation and temperature at the glacial-interglacial scale. Therefore, the sensitivity of TOC in lake sediments to climate change is different at different places or different time scales; it should be very careful to use lacustrine TOC as an index of Asian summer monsoon intensity, especially in the cold and high-altitude regions.

Centennial-scale climate variability during the past 2000 years derived from lacustrine sediment on the western Tibetan Plateau

The Tibetan Plateau (TP) climate change has been suggested to be modulated by the Asian monsoon and the westerly jet at seasonal, decadal, millennial and glacial-interglacial scales, which demonstrated evident regional variation. However, due to the lack of high resolution climate records, how the climate changed and how the regional environment was influenced by the monsoon and the westerlies on the western Tibetan Plateau (WTP) during the late Holocene remains unclear. Here we present multiple palaeo-climatic proxy records in sediment core from Xiada Co in the WTP for the past 2000 years, based on the relative distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), n-alkanoic acid average chain length (ACL) and percent aquatic inputs (Paq), XRF, total organic carbon (TOC) and inorganic carbon (TIC) concentration. The centennial-scale variation of the climate records captures well-known climatic events over the past 2000 years, for example, the cool-dry ‘Little Ice Age’ (LIA) and the warm-wet ‘Medieval Warm Period’ (MWP). Besides, the multi-proxy indicated a severe dry event during 350-250 cal yr BP and a cool-wet climate during the past 200 years. This study concludes that Xiada Co was mainly influenced by the Indian monsoon during the past 2000 years. In addition, the temperature variability appears to correspond to solar irradiance changes, suggesting a significant solar influence on temperature variability in the WTP. Our results suggest that the abrupt drought near 320 cal yr BP may be the key factor leading to the demise of the Guge Kingdom.

Distribution and isotopic composition of foraminifera at cold-seep Site 973-4 in the Dongsha area, northeastern South China Sea

A 1375-cm-long gravity core (Site 973-4) was acquired from the Dongsha cold seep area of the northeastern South China Sea (SCS). We measured its stable isotopes of planktonic foraminifera and investigated benthic foraminiferal assemblage compositions. Accelerator mass spectrometry 14C analysis of planktonic foraminifera shows that some intervals are dated to Marine Isotope Stage (MIS) 3. Pulleniatina obliquiloculata has positive δ13C (0.25–1.25‰) except for a negative carbon isotopic excursion (up to δ13C = −1.15‰), but its light-carbon sources remain elusive. The benthic δ13C values (Uvigerina) of non-seep and cold-seep boreholes from the SCS have no systematic difference. Therefore, the benthic δ13C at Site 973-4 did not record the enhanced seepage activities (if occurred) during the last sea-level lowstand in the Dongsha area. In the pre-Holocene sediments, the foraminiferal assemblages have lower diversities, Shannon-Wiener indices, evenness indices, Simpson indices and higher calcareous proportions like typical cold-seep settings. The changes in oxygen levels (lower in the glacial period than those in the Holocene) indicated by foraminiferal indices reflect either bottom-water oxygen variations on the glacial-interglacial scale or enhanced seepage activities during the last glacial period.

Provenance, sea-level and monsoon climate controls on silicate weathering of Yellow River sediment in the northern Okinawa Trough during late last glaciation

High resolution multi-proxy records, including geochemical and grain-size data from detrital sediments of IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for constraining sediment source and transport processes. They also allow silicate weathering and erosion controlled by sea-level change and East Asian summer monsoon evolution since 34ka to be reconstructed. Provenance proxies indicated that sediments in the northern Okinawa Trough were mainly supplied by the Yellow River middle reach at ~34–8ka. The low sea level and proximity of paleo-Yellow River mouth to the northern Okinawa Trough were the causes of the dominant Yellow River input. After ~8ka, a retreated Yellow River mouth coupled with the blocking effect of the Kuroshio Current and its branches, together with strong East Asian summer monsoon precipitation resulted in relatively strong sediment input from Kyushu to the northern Okinawa Trough. Meanwhile, Yellow River upper reach supplied more sediments to the study site than that during the ~34–8ka. On glacial-interglacial scale, silicate weathering proxies indicate that the core sediment was more weathered during the last glacial and deglacial than that in the modern Yellow River. We attribute this to the increased upper reach sediment input, which supplied more weak weathered sediment to the study site during the Holocene. Besides, composite effect of weathering during glacial and deglacial sediment production and additional weathering upon subaerial exposure of shelf deposits during low sea-level stage, as well as older weathered sediments reworking during sea-level rise could also account for this weathering regime. On multi-millennial scale, from ~34 to 18.5ka, silicate weathering was mainly controlled by the East Asian summer monsoon, with a cooling and drying climate associated with weakened summer monsoon activity. This is consistent with the reduced alteration of sediments eroded from the Yellow River basin. From ~18.5 to 8ka, strong reworking of older weathered sediments overwhelmed the East Asian summer monsoon in controlling silicate weathering, which induced a continuous increase in chemical alteration of the sediments during this period.

Hydroclimatic variability in loess δDwax records from the central Chinese Loess Plateau over the past 250 ka

This study reports hydrogen isotopic records from the central Chinese Loess Plateau (CLP) over the past 250 ka. After eliminating the influence of ice and local temperatures, the δDwax records extracted from two loess sites at Xifeng and Luochuan can be taken to represent arid/humid alternations in the hydrological environment in this marginal Asian Summer Monsoon (ASM) region; they also contain integrated information on summer precipitation patterns and the corresponding responses to these changes by predominant vegetation cover types. These arid/humid alternations show 100 ka, 40 ka and 20 ka cycles. An increase in precipitation in association with an enhanced summer monsoon has historically been taken to be the major factor driving a humid environment in the central CLP. However, hydroclimatic changes in δDwax records differ for the central CLP, central China and southern China. Over a 20 ka cycle, the influence of solar insolation on hydroclimatic changes can be shown to be consistent throughout the central CLP. However, changes in the relative location of the land and sea may have caused different hydroclimatic responses between southern China and the central CLP on a glacial-interglacial scale. The hydroclimatic variability in the central CLP would suggest that an enhanced summer monsoon due to climatic warming is the key to understanding decreased drought degree in this marginal monsoonal region.

Hydrogen isotope ratios of leaf wax n-alkanes in loess and floodplain deposits in northern China since the Last Glacial Maximum and their paleoclimatic significance

The hydrogen isotopic composition of leaf-wax n-alkanes (δDn-alkane) is increasingly used as a proxy for estimating δD of past precipitation (δDp). However, aridity can also affect sedimentary δDn-alkane, complicating the interpretation of paleo-δDn-alkane records. In order to evaluate the effects of the complex interactions and balance between precipitation hydrogen isotope composition and aridity on sedimentary δDn-alkane records, we present two δDn-alkane records from the North China Plain and the western Chinese Loess Plateau across a steep east-west climatic gradient in the East Asian Summer Monsoon area since the Last Glacial Maximum. The estimated δDp changes, based on a present-day relationship between δDp and rainfall amounts, can explain much of the δDn-alkane variations in the North China Plain, but do not explain the large variations of δDn-alkane in the western Chinese Loess Plateau. Comparison of δDn-alkane records with observed δDn-alkane values in modern plants and published pollen data indicates that changes in vegetation play a minor role in the variations of δDn-alkane. Instead, δDn-alkane values are coincident with aridity both temporally and spatially, i.e. the higher δDn-alkane values the drier climate and vice versa. Aridity effects are more evident in the western Chinese Loess Plateau than in the North China Plain. Therefore, on glacial-interglacial scale, sedimentary δDn-alkane records from the North China Plain mostly reflect changes in δDp values, whereas those from the western Chinese Loess Plateau largely reflect the effects of aridity, mainly due to a greater evapotranspirational deuterium-enrichment caused by a drier climate in the western Chinese Loess Plateau than in the North China Plain. Further calibrations from modern sediments are required to better define the quantitative relationship between the δDn-alkane and climate in northern China.

Influence of the Indian monsoon and the subtropical jet on climate change on the Tibetan Plateau since the late Pleistocene

Precipitation atop the Tibetan Plateau (TP) is delivered by the Indian summer monsoon, the Asian summer monsoon, and weather systems associated with the subtropical westerly jet. Variations in the relative importance of the monsoon systems and the westerly jet are hypothesized to have occurred at decadal, millennial and glacial-interglacial scales. However, paleoclimate observations based on explicit climate proxies are still scarce, limiting our understanding of the mechanisms of Holocene climate variability on the Tibetan Plateau (TP). Here we present three independently dated compound specific hydrogen isotope records of sedimentary leaf waxes from lakes on the TP, Bangong Co, Lake Qinghai and Linggo Co. The leaf wax δD records reflect isotopes in precipitation, and we combine these observations with existing isotopic and hydrological data to investigate variations in the influence of the summer monsoon and the westerly jet on the moisture budget of the TP since the Late Pleistocene. δD values of precipitation at all three lakes were relatively positive during the Late Pleistocene indicating a weakened summer monsoon. During the early and mid-Holocene, δD values of precipitation at the three lakes were relatively negative, suggesting the importance of summer monsoon. During the middle to late Holocene, δD values at Bangong Co and Lake Qinghai gradually increased with superimposed episodes of short term of δD variability. However, at Linggo Co in the northern TP, periods of more positive δD values of precipitation correspond to wetter intervals inferred from lake level high stands, and likely reflect variations in moisture associated with the westerly jet. Thus, the δD records at Linggo Co imply the lesser importance of summer monsoon moisture in the hydrologic budget of the northern TP. Collectively, the hydrogen isotope records at these three lakes document millennial and centennial scale variations in the strength of the summer monsoon systems and concurrent changes in the westerly jet. Furthermore, millennial-scale fluctuations in the δD records at the three lakes during the middle to late Holocene suggest episodes of reduced summer monsoonal moisture delivery to these regions, and correspond with intervals of cool sea surface temperatures in the North Atlantic.

Late Pleistocene-Holocene variability in the southern Gulf of Mexico surface waters based on planktonic foraminiferal assemblages

Abstract Planktonic foraminiferal assemblages are used as proxies of surface ocean variations, as documented in five marine sediment cores from the Bay of Campeche (BoC), Gulf of Mexico (GoM), spanning the last 34,000 cal yrs BP. This paper seeks to better understand the regional history by studying cores taken in the poorly studied Southern GoM. Multivariate factor analyses calculated five assemblages at millennial to glacial–interglacial scales. During the last glacial period, including the Last Glacial Maximum (LGM), the assemblage contained G. inflata , and T. sacculifer which suggests the presence of colder and less saline waters than younger times; although during the LGM, saltier waters and weaker ocean circulation may have occurred. During the deglaciation, the deep-dweller G. crassaformis suggest a reorganization of deep-thermocline waters and the establishment of transitional conditions, where mild to warm waters entered the Gulf. During the early to mid-Holocene, the tolerant tropical species G. ruber pink suggests salinity variations and warmer conditions than the previous interval. During the late-Holocene, only warm species comprise the assemblage, where the group G. menardii and G. truncatulinoindes suggest warm and stratified conditions, with an oxygen decrease in the thermocline, and rather high salinity levels linked to reduced moisture exported from the Pacific due to the progressively more influential ENSO-like conditions. Prominently, all the assemblages include the species G. ruber white chromotype, suggesting that the environmental conditions are continuously favorable to this abundant species. These scenarios show agreement to previous reconstructions in northern and western GoM, the Caribbean, as well as paleoclimate on land, which are mostly regulated by the position of the ITCZ, changes in the expansion of the Atlantic Warm Pool, the tropical circulation, and teleconnections with nearby regions.