Glacial-interglacial Timescales Research Articles

Increasing Pleistocene permafrost persistence and carbon cycle conundrums inferred from Canadian speleothems.

Permafrost carbon represents a potentially powerful amplifier of climate change, but little is known about permafrost sensitivity and associated carbon cycling during past warm intervals. We reconstruct permafrost history in western Canada during Pleistocene interglacials from 130 uranium-thorium ages on 72 speleothems, cave deposits that only accumulate with deep ground thaw. We infer that permafrost thaw extended to the high Arctic during one or more periods between ~1.5 million and 0.5 million years ago but has been limited to the sub-Arctic since 400,000 years ago. Our Canadian speleothem growth history closely parallels an analogous reconstruction from Siberia, suggesting that this shift toward more stable permafrost across the Pleistocene may have been Arctic-wide. In contrast, interglacial greenhouse gas concentrations were relatively stable throughout the Pleistocene, suggesting that either permafrost thaw did not trigger substantial carbon release to the atmosphere or it was offset by carbon uptake elsewhere on glacial-interglacial time scales.

Changes in sedimentary provenance and climate off the coast of Northeast Brazil since the Last Interglacial

Neodymium (Nd) and strontium (Sr) isotopic signatures in marine sediments reflect the geology of the associated drainage basin from which those sediments are supplied. These signatures are significantly affected by relief, weathering, transport, and deposition of sedimentary contributions from the respective sources. This work makes available new data of SrNd isotope ratios and Rare Earth Element (REE) concentrations, which provides a record of the sedimentary provenance of marine sediment supplied to the continental slope off the coast of northeastern Brazil since the last 130 thousand years. Our results indicate the main source of sediments to this site was the Parnaíba sedimentary basin. Furthermore, the fluctuations in Fe/K, Al/Si ratios, and Sr values are explained by geochemical processes operating within the region, which likely reflects changes in the hydrological regime and relative sea-level, and intensity of weathering in continental rocks in glacial-interglacial timescales. We suggest that low values of 87Sr/86Sr ratios during glacial are a result of dry conditions. Also, minor fluctuations within the last glacial period may be a consequence of displacements of the Intertropical Convergence Zone (ITCZ). Such displacements cause changes in rainfall and consequently variations in chemical weathering which affect Sr isotopic fractionation. During past humid intervals, the low Nd (non-radiogenic) signal observed could be explained as a process of penetration of rainwater into the deepest non-radiogenic sediment and subsequent replacement of the isotopic signal in the environment in glacial-interglacial timescales.

Geochronology and Geochemical Properties of Mid-Pleistocene Sediments on the Caiwei Guyot in the Northwest Pacific Imply a Surface-to-Deep Linkage

Seamounts are ubiquitous topographic units in the global ocean, and their effects on local circulation have attracted great research attention in physical oceanography; however, fewer relevant efforts were made on geological timescales in previous studies. The Caiwei (Pako) Guyot in the Magellan Seamounts of the western Pacific is a typical seamount and oceanographic characteristics have been well documented. In this study, we investigate a sediment core by geochronological and geochemical studies to reveal a topography-induce surface-to-bottom linkage. The principal results are as follows: (1) Two magnetozones are recognized in core MABC–11, which can be correlated to the Brunhes and Matuyama chrons; (2) Elements Ca, Si, Cl, K, Mn, Ti, and Fe are seven elements with high intensities by geochemical scanning; (3) Ca intensity can be tuned to global ice volume to refine the age model on glacial-interglacial timescales; (4) The averaged sediment accumulation rate is ~0.73 mm/kyr, agreeing with the estimate of the excess 230Th data in the upper part. Based on these results, a proxy of element Mn is derived, whose variability can be correlated with changes in global ice volume and deep-water masses on glacial-interglacial timescales. This record is also characterized by an evident 23-kyr cycle, highlighting a direct influence of solar insolation on deep-sea sedimentary processes. Overall, sedimentary archives of the Caiwei Guyot not only record an intensified abyssal ventilation during interglaciations in the western Pacific, but also provide a unique window for investigating the topography-induced linkage between the upper and bottom ocean on orbital timescales.

A ~130 ka terrestrial-marine interaction sedimentary history of the northern Jiangsu coastal plain in China

Coastal deposits are ideal archives to decipher the history of terrestrial-marine interactions in response to sea-level changes. Over the last decades, numerous investigations have been implemented to uncover the stratigraphic framework and the paleoenvironment evolution of the Jiangsu coastal plain (JCP), China. However, there have been limited studies on the sedimentary history over the glacial-interglacial timescales because of the lack of reliable chronostratigraphy. Here, we selected a 30-m-long sediment core in the northern JCP for luminescence dating, and carried out comprehensive sedimentological and paleontological analyses to ascertain the sedimentary history. Multiple luminescence dating techniques, including quartz optically-stimulated luminescence (OSL) and feldspar (polymineral) post-infrared (IR) IR stimulated luminescence (pIRIR) dating, were applied to determine the chronology of the sediment core. The obtained chronostratigraphy of core JCP01 spanning ~130 ka reveals that terrestrial deposition occurred in Marine Isotope Stage (MIS) 6, MIS 4–2 and the late stage of MIS 1. In contrast, marine influenced deposition is identified in MIS 5e and early MIS 1. The terrestrial interval of core JCP01 mainly consists of stiff clay and floodplain facies occurring at depths of 30–24 m, 12–5 m and 2–1 m, formed in relation to sea-level lowstands; whereas marine-influenced sediments at depths of 24–12 m and 5–2 m correspond to sea-level highstands, respectively. Our results highlight two sets of marine influenced sedimentary stratigraphy (T1, T2) dated to ~7 ka and around 130–120 ka, but no obvious MIS 3 transgression recorded in this core. In general, the conclusion is consistent with previous studies in East Asia and beyond, but not with previous work in this region that relied on radiocarbon dating. This suggests that re-dating with luminescence techniques is required for deposits correlated with the MIS 3 transgression in the northern JCP. Our reconstructed terrestrial-marine alternation sedimentary history of the northern JCP also shows a strong linkage with changes in the relative sea-level over the glacial-interglacial cycles.

Manganese co-limitation of phytoplankton growth and major nutrient drawdown in the Southern Ocean

Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients. Variability in utilization alters oceanic CO2 sequestration at glacial-interglacial timescales. The role for insufficient iron has been examined in detail, but manganese also has an essential function in photosynthesis and dissolved concentrations in the Southern Ocean can be strongly depleted. However, clear evidence for or against manganese limitation in this system is lacking. Here we present results from ten experiments distributed across Drake Passage. We found manganese (co-)limited phytoplankton growth and macronutrient consumption in central Drake Passage, whilst iron limitation was widespread nearer the South American and Antarctic continental shelves. Spatial patterns were reconciled with the different rates and timescales for removal of each element from seawater. Our results suggest an important role for manganese in modelling Southern Ocean productivity and understanding major nutrient drawdown in glacial periods.

Comparison of boosted regression trees vs WA-PLS regression on diatom-inferred glacial-interglacial climate reconstruction in Lake Tiancai (southwest China)

Quantitative paleoclimate reconstructions based on biological fossils over glacial-interglacial timescales are a major source of information on long-term climate variability. However, such reconstructions can present major methodological challenges, as calibration methods based on modern climatic and biological patterns may yield biased results, especially when data are particularly sensitive to the influence of secondary environmental variables in the training set or fossil assemblage. A machine-learning technique, boosted regression tree (BRT), is compared with a weighted averaging-partial least squares (WA-PLS) regression in a diatom-based summer (mean July) temperature transfer function with a calibration set of 273 samples collected from China. Using these calibration methods and a fossil diatom sequence from Lake Tiancai (southwest China; 3898 m a.s.l.), the summer temperatures covering the last glaciation and entire Holocene (18000 cal yr BP to present) are reconstructed. The record is validated by applying several statistical reconstruction diagnostics and compared with pollen and diatom records from the same sediment core. The results suggest that the BRT-based calibration model, which produced a coefficient of determination (r2 = 0.85) and root-mean-square error of prediction (RMSEP = 1.65 °C) offers substantial improvements over WA-PLS. The record shows that the summer temperature varies by ~2.5 °C across the entire period. The temperature evolution inferred by diatom-based BRT model is characterized by an early cool phase (with a MJT of 9.3 °C) prior to 10000 cal yr BP, followed by a clear mid-Holocene thermal maximum (~1.1 °C above the present-day temperature) from 6000 to 3500 cal yr BP and a subsequent cooling that ended at 1500 cal yr BP. This contrasts with the chironomid-based temperature transfer function in the same sequence showing a warmer period between 8500 and 6000 cal yr BP. The overall pattern of diatom-based summer temperature reconstructions broadly matches the climate change inferred from pollen records from Tiancai Lake and other records from the surrounding regions, suggesting that summer temperatures respond to insolation forcing and the variability of the southwest monsoon. We argue that the BRT-based climate model potentially outperforms the other models in southwest China when using a large and non-linear calibration set of diatoms for reconstructing long-term temperature.

Glacial-interglacial variation in catchment weathering and erosion paces the Indian summer monsoon during the Pleistocene

Silicate weathering via CO2 consumption is considered to regulate the long-term climate change. Discriminating silicate versus carbonate weathering from sedimentary records is helpful to understand the coupling relationship between weathering and past climate. However, owing to the paucity of well-dated and continuous records of weathering and erosion, the relationship between silicate and carbonate weathering and their links with mountain uplift and glacial-interglacial climate remain elusive. Geochemical analysis of a 666 m core from the Heqing basin, a paleolake situated at the southeastern margin of the Tibetan Plateau, demonstrates silicate versus carbonate weathering and erosion changes under Indian summer monsoon (ISM) conditions spanning the entire Pleistocene, via respective tracing between Rb/Sr and Al/Na ratios. The Rb/Sr record is interpreted as reflecting a competition between clayey silicate-origin Rb versus carbonate-origin Sr fluxes, whereas Al/Na ratio is a balance between weathering and erosion of silicate minerals, both pacing the glacial-interglacial cycles of the ISM. During interglacials, strong silicate weathering intensity reflected by low Al/Na values is mainly controlled by warmer temperatures, with the reduced variability during 1.82–0.92 Ma, recalling the pacing of the Arabian Sea surface temperature amplitudes. Within glacials, the weak carbonate leaching and clay erosion within the older and younger intervals are interrupted by strong clay erosion within the middle interval. In conclusion, the Heqing Rb/Sr and Al/Na records reveal the sensitivity of catchment weathering and erosion to ISM changes over glacial-interglacial timescales with warmer temperature enhancing silicate weathering and clay formation during interglacials and greater physical erosion during glacials.

The sedimentary record of Quaternary glacial to interglacial sea‐level change on a subtropical carbonate ramp: Southwest Shelf of Australia

AbstractIn the last decades, the understanding of temperate carbonate systems has improved considerably, but their development over glacial–interglacial timescales is still understudied in comparison to their tropical counterparts. A key question is how do temperate carbonate platforms respond to high‐amplitude, glacial–interglacial sea‐level changes? Integrated Ocean Drilling Program Site U1460 was drilled at the uppermost slope of the Southwest Shelf of Australia at the transition between the subtropical Carnarvon Ramp and the warm–temperate Rottnest Shelf. The origin and composition of the sediments in the upper 25 m below seafloor at Site U1460 were investigated using X‐ray diffraction, scanning electron, and light microscopy. The Middle Pleistocene to Holocene sequence at Integrated Ocean Drilling Program Site U1460 contains a record of sea‐level controlled sedimentary cycles. Carbonate sediments deposited during interglacial sea‐level highstands (Marine Isotope Stages 1, 5, most of 7, 9 and 11) are mainly fine‐grained (<63 µm) and dominated by low‐Mg calcite from pelagic bioclasts such as planktic foraminifera. The glacial lowstand intervals (Marine Isotope Stages 2 to 4, 6, 8, 7d, 10 and 12), instead are coarser‐grained and relatively rich in aragonite and high‐Mg calcite from neritic bioclasts, such as bryozoans. These changes in texture, mineralogy and composition are best explained by the deposition of neritic bioclasts closer to the shelf edge during glacial sea‐level lowstands. During early transgression, reworking of bioclast‐rich coastal dune deposits likely leads to transport and redeposition of neritic clasts on the upper slope. In contrast, dominantly pelagic sediments characterize deposition at the platform edge during interglacial highstands. These results highlight regional differences in the response of temperate carbonate systems to sea‐level change: A previously published model developed for early Pleistocene temperate carbonates from the Great Australian Bight indicates that shelfal material was exported to the upper slope during sea‐level highstands. It is argued that this difference is related to the change in duration and amplitude of glacial–interglacial sea‐level cycles before and after the Mid‐Pleistocene transition.

The distribution of (234U/238U) activity ratios in river sediments

Uranium (U) isotopes can be used to estimate the comminution age of sediments, i.e. the time elapsed from sediment production on continents, via weathering and physical erosion, to deposition in the sedimentary record. The calculation of this comminution age is based on measured (234U/238U) activity ratios in river sediments, and inferred time-dependent recoil effect, which leads to the preferential release of 234U from mineral lattices during erosion processes. In this study, we report on a large-scale (234U/238U) investigation of modern river sediments worldwide, with the aim to determine the extent to which parameters such as grain size, lithology, weathering, climate and geomorphology may influence the distribution of U isotopes in fine-grained sediments. Our extensive dataset (N = 64) includes U isotopic measurements for many of the world’s largest rivers, but also rivers draining particular climatic and geological settings. Our results indicate that sediments collected from river basins draining mostly igneous, metamorphic or volcanic rocks often display (234U/238U) ratios > 1, with clay-size fractions (<4 µm) being less depleted in 234U (higher 234U/238U) than corresponding silt-size fractions (4–63 µm). In contrast, sediments derived from multi-lithological basins or draining sedimentary rocks are typically characterized by (234U/238U) ratios <1, with clays generally exhibiting more depleted 234U signatures than silts. Taken together, these observations suggest that the formation of secondary clay minerals in soils from basins draining mostly igneous, metamorphic is accompanied by partial incorporation by recoil injection of 234U initially released during weathering processes, possibly from U-rich minerals, such as sphene or apatite. Instead, in multi-lithological catchments draining sedimentary rocks, we propose that the erosion of recycled sediments having experienced several cycles of weathering, possibly over glacial-interglacial timescales, could explain the much lower (234U/238U) ratios observed in clay-size fractions. While no direct relationships can be identified between sediment (234U/238U) ratios and lithology, weathering intensity, climatic or geomorphic parameters in corresponding river basins, we show that the catchment size probably plays an important role in controlling the distribution of (234U/238U) in river sediments, through its direct influence on the sediment residence time. Finally, a multiple regression analysis of our data, combining various environmental parameters for the lithology, climate and geomorphology of studied river basins, indicates predicted (234U/238U) values that are very similar to measured values (with R2 ∼ 0.8). This finding provides further support for the usefulness of (234U/238U) ratios in the sedimentary record for reconstructing past landscape changes and their effect on sediment transport and residence time in river basins.

Complex response of weathering intensity registered in the Andaman Sea sediments to the Indian Summer Monsoon over the last 40 kyr

Downcore geochemical composition of sediment core retrieved from the central Andaman Sea were measured to help understanding the potential connection between chemical weathering and monsoon climate since the last glacial period. The age model was confirmed based on twelve accelerator mass spectrometry (AMS) 14C dates on planktonic foraminifera Neogloboquedrina dutertrei. Provenance discrimination results distinguished by Rare Earth Elements pattern and Sc/Th-(La/Yb)UCC diagram revealed that terrestrial sediment from the Irrawaddy River are the dominant input in the central Andaman Sea. Chemical weathering indexes, including chemical index of alteration (CIA), weathering index of Parker (WIP), αAlNa, and a stack of these data were reported and the terrestrial input was assessed using Ti/Ca ratios. These indexes showed general synchronicity with the regional monsoon precipitation and temperature record, the chemical weathering intensity and monsoon precipitation/temperature were stronger/higher during MIS 3 and MIS 1 and weaker/lower during MIS 2, indicating existence of a general “climate-driving weathering” pattern on glacial-interglacial time scales. However, certain anomalies occurred during 18–13 cal. kyr BP and 6–3 cal. kyr BP, when the chemical weathering intensity showed higher and lower values, respectively. The chemical weathering intensity enhanced during 18–14 cal. kyr BP, due to increased isolation and markedly increased temperatures, as well as lower erosion production and sediment transfer rate caused by lower precipitation, whereas the opposite conditions prevailed during 14–13 cal. kyr BP. Indeed, for the weak chemical weathering intensity during 6–3 cal. kyr BP, decreased isolation, relatively high terrestrial sediment supply, slightly lower precipitation and markedly lower temperatures were suggested to be the primary reasons, indicating the presence of a “climate-driving weathering” pattern on millennial time scales. Our findings support the hypothesis that the chemical weathering evolution in the north-eastern Indian Ocean was synchronous with both global and regional climate conditions, representing distinct changes controlled by Indian summer monsoon on different time scales.

Modeling the evolution of pulse-like perturbations in atmospheric carbon and carbon isotopes: the role of weathering–sedimentation imbalances

Abstract. Measurements of carbon isotope variations in climate archives and isotope-enabled climate modeling advance the understanding of the carbon cycle. Perturbations in atmospheric CO2 and in its isotopic ratios (δ13C, Δ14C) are removed by different processes acting on different timescales. We investigate these differences on timescales of up to 100 000 years in pulse-release experiments with the Bern3D-LPX Earth system model of intermediate complexity and by analytical solutions from a box model. On timescales from years to many centuries, the atmospheric perturbations in CO2 and δ13CO2 are reduced by air–sea gas exchange, physical transport from the surface to the deep ocean, and by the land biosphere. Isotopic perturbations are initially removed much faster from the atmosphere than perturbations in CO2 as explained by aquatic carbonate chemistry. On multimillennial timescales, the CO2 perturbation is removed by carbonate compensation and silicate rock weathering. In contrast, the δ13C perturbation is removed by the relentless flux of organic and calcium carbonate particles buried in sediments. The associated removal rate is significantly modified by spatial δ13C gradients within the ocean, influencing the isotopic perturbation of the burial flux. Space-time variations in ocean δ13C perturbations are captured by principal components and empirical orthogonal functions. Analytical impulse response functions for atmospheric CO2 and δ13CO2 are provided. Results suggest that changes in terrestrial carbon storage were not the sole cause for the abrupt, centennial-scale CO2 and δ13CO2 variations recorded in ice during Heinrich stadials HS1 and HS4, though model and data uncertainties prevent a firm conclusion. The δ13C offset between the Penultimate Glacial Maximum and Last Glacial Maximum reconstructed for the ocean and atmosphere is most likely caused by imbalances between weathering, volcanism, and burial fluxes. Our study highlights the importance of isotopic fluxes connected to weathering–sedimentation imbalances, which so far have been often neglected on glacial–interglacial timescales.

Hypersensitivity of glacial summer temperatures in Siberia

Abstract. Climate change in Siberia is currently receiving a lot of attention because large permafrost-covered areas could provide a strong positive feedback to global warming through the release of carbon that has been sequestered there on glacial–interglacial timescales. Geological evidence and climate model experiments show that the Siberian region also played an exceptional role during glacial periods. The region that is currently known for its harsh cold climate did not experience major glaciations during the last ice age, including its severest stages around the Last Glacial Maximum (LGM). On the contrary, it is thought that glacial summer temperatures were comparable to the present day. However, evidence of glaciation has been found for several older glacial periods. We combine LGM experiments from the second and third phases of the Paleoclimate Modelling Intercomparison Project (PMIP2 and PMIP3) with sensitivity experiments using the Community Earth System Model (CESM). Together, these climate model experiments reveal that the intermodel spread in LGM summer temperatures in Siberia is much larger than in any other region of the globe and suggest that temperatures in Siberia are highly susceptible to changes in the imposed glacial boundary conditions, the included feedbacks and processes, and to the model physics of the different components of the climate model. We find that changes in the circumpolar atmospheric stationary wave pattern and associated northward heat transport drive strong local snow and vegetation feedbacks and that this combination explains the susceptibility of LGM summer temperatures in Siberia. This suggests that a small difference between two glacial periods in terms of climate, ice buildup or their respective evolution towards maximum glacial conditions can lead to strongly divergent summer temperatures in Siberia, allowing for the buildup of an ice sheet during some glacial periods, while during others, above-freezing summer temperatures preclude a multi-year snowpack from forming.

Clumped and stable isotopes of land snail shells on the Chinese Loess Plateau and their climatic implications

We report stable (δ13Cshell, δ18Oshell) and clumped isotope (Δ47) compositions of modern and last glacial fossil snail shell carbonates from the Luochuan and Weinan sections on the central and southern Chinese Loess Plateau (CLP). Our study reveals that the average Δ47 temperature (T47) of modern snails is consistent with monitored temperatures during the snail growing season at the studied locations and is ~10 °C higher than that of fossil snails from glacial time. Moreover, the average δ13Cshell of modern snails is more depleted than that of fossils. We argue that the δ13Cshell cannot record changes in plant communities (i.e., the C3/C4 ratio) on the CLP and may mainly indicate arid conditions with depleted values reflecting reduced aridity. Additionally, the reconstructed snail body water δ18O (δ18Owater) of modern snails is more enriched than δ18O in modern growing season precipitation and δ18Owater of fossils. This contrast may be related to the high degree of evaporative enrichment of environmental water 18O in the body/ingested by modern snails under warm conditions. Therefore, we suggest that using δ18Oshell to directly reconstruct the oxygen isotopes of precipitation is difficult and that higher δ18Oshell and δ18Owater values probably indicate higher environmental temperature/stronger evaporative enrichment on glacial-interglacial timescales on the CLP.

Climates of the last three interglacials in subtropical eastern Australia inferred from wetland sediment geochemistry

Records of Australian climate during Marine Isotope Stages 5 and 7 (130–71 and 243–191 ka) are rare, preventing detailed assessments of long-term climate, drivers and ecological responses across the continent over glacial-interglacial timescales. This study presents a geochemistry-based palaeoclimate record from Fern Gully Lagoon on North Stradbroke Island (also known as Minjerribah) in subtropical eastern Australia, which records climates in MIS 7a–c, MIS 5 and much of the Holocene, in addition to MIS 4 (71–57 ka), and parts of MIS 6, MIS 3 and MIS 2 (191–130, 57–29 and 29–14 ka). Indicators of inorganic sedimentation from a 9.5 m sediment core – focussed on high-resolution estimates of sediment geochemistry supported by x-radiography, inorganic content and magnetic susceptibility – were combined with a chronology consisting of six radiocarbon (14C) and thirteen single-grain optically stimulated luminescence (OSL) ages. Hiatuses occurred at ~178–153 ka, ~36–21 ka and ~7–2 ka and likely result from the wetland drying. Low values of locally sourced aeolian materials indicate a wet MIS 7a–c and early MIS 6 before a relatively dry MIS 5. Inorganic flux during the Holocene was up to four times greater than during MIS 5, consistent with long-term interglacial drying observed in other regions, most notably in central Australia. This study highlights the importance of employing a combination of multiple dating approaches and calibrated geochemical proxies to derive climate reconstructions and to identify depositional complexities in organic-rich wetland records.

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.

Permafrost-related hiatuses in stalagmites: Evaluating the potential for reconstruction of carbon cycle dynamics

Abstract Permafrost is widely present throughout the Northern Hemisphere high latitudes, and stores large amounts of carbon in the form of frozen soil organic matter. The response of permafrost regions to anthropogenic climate change remains uncertain, in part because of a lack of information on their response to past changes in global climate. Here we test the use of stalagmites from two caves in Siberia as a novel, precisely dated, and highly localised archive of past permafrost carbon cycle dynamics. Stalagmite growth at these sites is controlled by the presence/absence of permafrost above the cave over glacial-interglacial time scales. We target the transition layer between two subsequent growth phases (interglacials) and the interval directly following growth resumption after the last glacial in three stalagmites, as this is where a geochemical imprint of thaw-related processes in the frozen zone between surface and cave would be recorded. We apply a multi-proxy approach including carbon isotopes (δ13C and 14C) and trace element concentrations, combined with petrographic analyses and high-resolution U-Th chronology. Our dataset indicates complex growth patterns and possible intervals of microbial colonisation of the stalagmite surface in the transition layers. High-resolution U-Th ages confirm that the transition layer is not a single, long growth hiatus, but rather a period of extremely slow or episodic growth phases, possibly during “skipped” interglacials. However, we find no conclusive evidence for a geochemical signature related to permafrost degradation and related local carbon cycle dynamics, which might be related to insufficient sensitivity of the archive for high-frequency processes and/or insufficient measurement resolution.

Equatorial Pacific seawater pCO2 variability since the last glacial period

The ocean may have played a central role in the atmospheric pCO2 rise during the last deglaciation. However, evidence on where carbon was exchanged between the ocean and the atmosphere in this period is still lacking, hampering our understanding of global carbon cycle on glacial–interglacial timescales. Here we report a new surface seawater pCO2 reconstruction for the western equatorial Pacific Ocean based on boron isotope analysis—a seawater pCO2 proxy—using two species of near-surface dwelling foraminifera from the same marine sediment core. The results indicate that the region remained a modest CO2 sink throughout the last deglaciation.

Evidence for wetter climate recorded in the Jingxian red clay section since approximately 840 ka ago and its relationship with the East Asian summer monsoon intensity

The Quaternary red clay (QRC) is one of the typical terrestrial sediments in southern China which contains abundant palaeoclimatic information. A natural QRC outcrop, named the Jingxian red clay section (JRCS), was developed in Jingxian City on the northern piedmont of Mt. Huangshan, offering a valuable archive for long-term palaeoclimatic study from a southern China perspective. The objective of this study is to reveal palaeoclimatic information of regional precipitation from the JRCS and its correlation with the East Asian summer monsoon (EASM) intensity, using grain-size measurements combined with dating via Electron Spin Resonance (ESR) and palaeomagnetism. The results indicate that: (1) Ten palaeoclimatic wetter periods (PWPs) designated as PWPI to PWPX from top to bottom were detected from the grain-size analysis results of the upper 9.4 m JRCS. These PWPs span a period from approximately 840 ka ago to present; (2) three PWPs (PWPVIII to PWPX) were basically determined to approximately 612–690, 720–762 and 792–822 ka ago, basically corresponding to China loess layers of L6, L7 and L8 and deep-sea isotope stages of MIS16, MIS18 and MIS20, respectively. Data suggests that the PWPs in the study area were observed mainly during glaciations; (3) regional precipitation in the study area should be negatively related to the EASM intensity since approximately 840 ka ago as a result of variations in North Hemisphere solar insolation and uplifts of the Tibetan Plateau; (4) the modern anti-phase of monsoonal rainfall at glacial-interglacial timescales seemed to have existed between northern China and southern China since the late Early Pleistocene (approximately 840 ka ago).

Competing droughts affect dust delivery to Sierra Nevada

The generation and transport of mineral dust is strongly related to climate on seasonal, year-to-year, and glacial-interglacial timescales. The modern dust cycle is influenced by soil moisture, which is partly a function of drought duration and severity. The production and transport of dust can therefore be amplified by global and regional droughts, thereby moderating ecosystem vulnerability to disturbance through the influence of dust on nutrient delivery to ecosystems. In this work, we use strontium and neodymium isotopes in combination with trace element concentrations in modern dust samples collected in 2015 to quantify the role of regionally versus globally supplied dust in nutrient delivery to a montane ecosystem. The study sites lie along an elevational transect in the southern Sierra Nevada, USA, with samples spanning the dry seasons of 2014 (Aciego et al., 2017) and 2015 (this study), when the region was experiencing a historic drought. The goal of our research was to quantify the spatial and temporal variability and sensitivity of the dust cycle to short term changes at nutrient-limited sites. We find that, during the dry season of 2015, Asian sources contributed between 10 and 40% of dust to sites located along this elevational transect, and importantly increased in importance during the summer growing season compared to regional dust sources. These changes are likely related to the prolonged drought in Asia in 2015, highlighting both the sensitivity of dust production and transport to drought and the teleconnections of dust transport in terrestrial ecosystems.

Palaeoproductivity and associated changes in the north-eastern Indian Ocean since the last glacial: Evidence from benthic foraminifera and stable isotopes

Here we present the first detailed planktonic and benthic δ13C records and benthic foraminiferal assemblage records from the north-eastern Indian Ocean to decipher the palaeoceanographic changes during the last 56 kyr. We identified three different palaeoceanographic stages, clearly differentiated by significant variations in the benthic foraminiferal assemblages and δ13C records. The results of this study indicate that productivity was generally higher during the glacial periods than during the Holocene. Comparison of the benthic foraminiferal assemblage distributions and planktonic and benthic δ13C records show a significant correlation between productivity and the bottom water oxygenation on glacial-interglacial timescales. Productivity gradually increased during the period between 56 and 27.5 kyr. During this period, the dominance of Melonis spp. and Oridosalis umbonatus were correlated with conditions of intermediate to high surface productivity and moderate bottom water oxygenation. Increased higher equatorial productivity and low bottom water oxygenation during the period between 27.5 and 15 kyr are supported by planktonic δ13C and faunal records. During this period, the dominance of benthic foraminifera assemblages characterised by Uvigerina peregrina indicates sustained continuous phytodetritus flux to the seafloor from enhanced surface water productivity and relatively low bottom water oxygenation. The absence or minimal occurrences of high productivity indicating U. peregrina, the dominance of intermediate to low productivity indicating fauna, and relatively low planktonic δ13C records suggest low productivity and active deep-water oxygenation after 15 kyr. Concurrent river discharge and rising sea levels during this period are indicated by negative δ18O of G. ruber at the site.