Kyr BP Research Articles

South Georgia marine productivity over the past 15 ka and implications for glacial evolution

Abstract. The subantarctic islands of South Georgia are located in the Southern Ocean, and they may be sensitive to future climate warming. However, due to a lack of well-dated subantarctic palaeoclimate archives, there is still uncertainty about South Georgia's response to past climate change. Here, we reconstruct primary productivity changes and infer Holocene glacial evolution by analysing two marine gravity cores: one near Cumberland Bay on the inner South Georgia shelf (GC673: ca. 9.5 to 0.3 cal. kyr BP) and one offshore of Royal Bay on the mid-shelf (GC666: ca. 15.2 cal. kyr BP to present). We identify three distinct benthic foraminiferal assemblages characterised by the dominance of Miliammina earlandi, Fursenkoina fusiformis, and Cassidulinoides parkerianus that are considered alongside foraminiferal stable isotopes and the organic carbon and biogenic silica accumulation rates of the host sediment. The M. earlandi assemblage is prevalent during intervals of dissolution in GC666 and reduced productivity in GC673. The F. fusiformis assemblage coincides with enhanced productivity in both cores. Our multiproxy analysis provides evidence that the latest Pleistocene to earliest Holocene (ca. 15.2 to 10.5 cal. kyr BP) was a period of high productivity associated with increased glacial meltwater discharge. The mid–late Holocene (ca. 8 to 1 cal. kyr BP), coinciding with a fall in sedimentation rates and lower productivity, was likely a period of reduced glacial extent but with several short-lived episodes of increased productivity from minor glacial readvances. The latest Holocene (from ca. 1 cal. kyr BP) saw an increase in productivity and glacial advance associated with cooling temperatures and increased precipitation which may have been influenced by changes in the southwesterly winds over South Georgia. We interpret the elevated relative abundance of F. fusiformis as a proxy for increased primary productivity which, at proximal site GC673, was forced by terrestrial runoff associated with the spring–summer melting of glaciers in Cumberland Bay. Our study refines the glacial history of South Georgia and provides a more complete record of mid–late Holocene glacial readvances with robust chronology. Our results suggest that South Georgia glaciers were sensitive to modest climate changes within the Holocene.

Holocene alluvial dynamics, soil erosion and settlement in the uplands of Macedonia (Greece): New geoarchaeological insights from Xerolakkos in Grevena

This paper addresses the interplay between Holocene landscape evolution and human settlement dynamics, drawing new evidence from the alluvial history of Xerolakkos, a continental stream in Grevena (Western Macedonia, Greece). We developed an integrated geoarchaeological survey combining remote sensing geomorphological mapping, litho-stratigraphic analysis and radiocarbon dating with the site evidence of a new archaeological survey. Results revealed four major alluviation phases, corresponding to 1) the beginning of the Holocene until the Early Neolithic (∼6300/6200 BCE), 2) the end of the Early and the Middle Neolithic (∼6000–5400 BCE), 3) from the Middle Bronze Age to the Late Roman period (∼1800 BCE – 500 CE), and 4) during the Byzantine and Ottoman eras (∼500–1800 CE), all separated by phases of floodplain incision. Furthermore, the effects of several Holocene Rapid Climatic Changes (RCC) are traced and discussed together with potential human responses; we also provide the first alluvial sequence recording the ∼6200 BCE (8.2 kyr BP) event in the Balkans. While the climate and the local geomorphological setting are considered the primary drivers behind instability and erosion during the Early and Middle Holocene, a landscape change starting in the Middle Bronze Age (after ∼1800 BCE) followed by a re-organisation of the rural economy in the Roman period suggests the increasing involvement of anthropogenic forcing which, by the Ottoman period, evolved into a dynamic situation between climatic variability and adaptive land management. Finally, we demonstrate how soil erosion in the upper catchment constitutes a serious taphonomic bias when studying the regional archaeological record.

Late Holocene relative sea-level records from coral microatolls in Singapore

Late Holocene relative sea-level (RSL) data are important to understand the drivers of RSL change, but there is a lack of precise RSL records from the Sunda Shelf. Here, we produced a Late Holocene RSL reconstruction from coral microatolls in Singapore, demonstrating for the first time the utility of Diploastrea heliopora microatolls as sea-level indicators. We produced 12 sea-level index points and three marine limiting data with a precision of < ± 0.2 m (2σ) and < ± 26 years uncertainties (95% highest density region). The data show a RSL fall of 0.31 ± 0.18 m between 2.8 and 0.6 thousand years before present (kyr BP), at rates between − 0.1 ± 0.3 and − 0.2 ± 0.7 mm/year. Surface profiles of the fossil coral microatolls suggest fluctuations in the rate of RSL fall: (1) stable between 2.8 and 2.5 kyr BP; (2) rising at ~ 1.8 kyr BP; and (3) stable from 0.8 to 0.6 kyr BP. The microatoll record shows general agreement with published, high-quality RSL data within the Sunda Shelf. Comparison to a suite of glacial isostatic adjustment (GIA) models indicate preference for lower viscosities in the mantle. However, more high quality and precise Late Holocene RSL data are needed to further evaluate the drivers of RSL change in the region and better constrain GIA model parameters.

East Asian summer monsoon and early anthropogenic impacts on the lacustrine carbon cycle in the Otindag Sandy Land since the middle Holocene

Lake systems play a pivotal role in the global carbon cycle, yet the biogeochemical processes in alkaline and saline lakes have received little attention. In this study, we present a comprehensive analysis of δ13Ccarb and δ18Ocarb records from Lake Chagan in the Otindag Sandy Land, located in North China, spanning approximately 7000 years. Our aim is to explore how the lacustrine inorganic carbon cycle responded to both climate change and human activity. The findings reveal a notable highstand in Lake Chagan around 6.9–6.0 cal kyr BP, followed by a gradual regression leading to an unstable state over the past 1.6 cal kyr BP. The hydroclimate variations inferred from Lake Chagan align with other regional records, suggesting that long-term East Asian summer monsoon (EASM) precipitation trends were influenced by decreasing insolation and variations in solar irradiance on a centennial timescale. The sources of dissolved inorganic carbon (DIC) in lake water generally mirrored regional hydroclimate changes since the middle Holocene. Our evidence indicates that during periods of strong EASM activity, more allochthonous organic matter was converted to lake water DIC, coinciding with high vegetation cover and intense soil erosion. While hydroclimatic changes significantly impacted the lacustrine carbon cycle, the isotopic evidence also suggests substantial anthropogenic influences post-1.6 cal kyr BP, particularly in terms of the input of allochthonous organic matter into the lakes of the Otindag Sandy Land. These findings underscore the potential for anthropogenic-driven terrestrial carbon cycle feedback to global climate change over millennia.

Benthic foraminiferal response to the bottom water oxygenation of the Southern Bay of Bengal during the last 44 kyr BP

Benthic foraminiferal response to the bottom water oxygenation of the Southern Bay of Bengal during the last 44 kyr BP

Tracing the Late Quaternary coastal evolution of Central Kerala, India, around the lost ancient port Muziris using multi-proxy study of the sedimentary archives

The Central Kerala coast in Southwest India serves as a valuable natural archive for studies of the Late Quaternary coastal evolution, palaeoclimate, and maritime trades. The coast is an archeologically and culturally significant region in South India, located in the mouth of the longest river in Kerala, the Periyar River. The riverbank of the Periyar at its mouth is believed to host the ancient port Muziris, whose remnants are found in the areas around where the Periyar River joins the Arabian Sea. Sea level oscillations and climate variabilities in the past have played a pivotal role in the destruction of the Muziris port. Recent excavations unearthed many artifacts from nearby areas of the river confluence; however, the exact location of the port remains unresolved. The present study is an attempt to trace out the coastal evolutionary processes and palaeoclimatic conditions that prevailed in the area during the Late Quaternary period using three drilled cores retrieved from this stretch of the coastal lands. A multi-proxy approach, combining geochemical, mineralogical, palynological, and geochronological tools was used in the study. The Holocene deposit in the study area is composed mainly of sand and clay-dominant sediments that fall within an age range of 1.1–8.1 cal kyr BP, whereas the underlying sedimentary sequence dates to a maximum of 31.5–37.9 cal kyr BP. The inorganic and organic elemental compositions, together with the textural and palynological attributes, indicate that Holocene sedimentation occurred here under fluctuating environmental conditions with significant changes in climate and sea level positions. The upper part of this section bears the signature of the humid depositional environment that prevailed during the Late Holocene when sand-dominated sediments were deposited and brought in under the influence of longshore drift. The sediments from the lower part of the section carry the signature of the Holocene transgression when deposition took place under submerged conditions with considerable marine influence but receiving significant terrestrial inputs. The study highlights the location of the historically significant lost ancient port of Muziris at Kodungallur near the mouth of the Periyar River. A three-fold evolutionary model proposed for the coastal segment revealed that the location of the Muziris port was part of the Periyar River near Kodungallur, as mentioned by the historians, and the settlement near Pattanam may be considered a satellite township of the ancient Muziris port.

A HOMEMADE SEMIAUTOMATIC GRAPHITIZATION DEVICE FOR AMS 14C DATING AT NTUAMS LAB

ABSTRACT A low-cost and computer-controlled graphitization system connected to an elemental analyzer (EA) has been designed and built at the NTUAMS Lab. This semiautomatic system equips 6-unit reactors for the graphitization of CO2 with H2 on the iron catalyst. The entire procedure takes about 7 hours for iron conditioning, sample combustion and loading, and graphitization. The system can produce good-quality graphite for samples containing 0.5–1.6 mg carbon mass, with the pressure yield of graphitization ranging from 57.7% to 87.1%. The average values of OXI and OXII agree well with the consensus value, but the result of ANU sucrose was observed to be slightly higher than the reported one. The background samples of anthracite over ten months yielded an average of 0.38±0.10 pMC (n=21) corresponding to a 14C age of 45 kyr BP. Intercomparison samples L and M of FIRI exhibit that the measured 14C ages are almost identical to the consensus values and have a small spread in these values. The system has been carrying out graphitization for total organic carbon (TOC) of peat samples, and providing a more efficient and convenient way for AMS 14C dating.

Asian monsoon variations over the past 21 ka: An introduction

The Asian monsoon influences the lives of more than a third of the world's population, and understanding its spatial and temporal variability is fundamental to sustainable development. The complex dynamics of this climatic system are therefore of broad scientific and societal interest. In this virtual special issue (VSI), we present 19 papers dealing with the Asian monsoon since the Last Glacial Maximum (LGM, ∼21 kyr BP), with a focus on understanding variability and forcing mechanisms. Findings use recently developed quantitative reconstructions combined with numerical simulations, and present novel insights as follows: (1) analyses of comprehensive datasets of pollen, diatom, brGDGTs, oxygen and hydrogen isotopes and frequency magnetic susceptibility, as well as an innovative algorithm model for the lake energy-water balance, improve the reliability of quantitative reconstructions of Asian monsoon climate variations; (2) studies of quantitative climate parameters in central East Asia show that mean annual temperature (MAT) was ∼5 °C (or ∼ 7.9 °C using brGDGTs) lower during the LGM, ∼1.5 °C (±41% using brGDGTs) higher around the Holocene thermal optimum (HO) and mean annual precipitation (MAP) varied between 30 and 150% with marked seasonality during 8–5 ka; (3) dynamical downscaling palaeoclimatic simulations improve the accuracy of past monsoon variations, revealing strong variability in monsoon precipitation across the LGM and HO. The holistic study of Asian monsoon variations at orbital to centennial timescales in these VSI papers presents reliable observations of monsoon variability and its driving mechanisms, providing high-resolution quantitative paleoclimate reconstructions that fill existing gaps in spatial coverage, as well as offering an improved understanding, thorough numerical simulation, of coupled sea-level change and vegetation feedbacks.

MIS5-MIS3 Neanderthal occupations at Amalda III cave (Northern Iberian Peninsula)

The arrival, occupation, and disappearance of Neanderthals in the Cantabrian region is the subject of a long-running debate that continues to this day. Knowledge of the evolution of Neanderthal societies since the end of the Middle Pleistocene in the north of the Iberian Peninsula is greatly impacted by the scarcity of a good chronological framework. This absence of good data creates difficulties in evaluating the cultural adaptations of these populations to environmental changes and their historical dynamics.The Amalda III cave, surveyed in 1983-84, contains a sequence of occupations ranging from the end of MIS5 (Levallois with Mousterian points and bifaces) to the beginning of MIS3 (Vasconian). It is possible to observe substantial changes in the environmental conditions and cultural responses of Neanderthal groups that inhabited the cave. Here we present the first results of the Amalda III sequence, which is currently being excavated by an interdisciplinary team.Our objective is to test, enrich and refine the regional environmental and Neanderthal cultural framework and the transformations that took place between the MIS5 and the MIS3. These new data are compared with the information available from several Middle Paleolithic sites in the region, advancing our understanding of the behavioural adaptations of Neanderthals to different ecological and socio-cultural situations between ca. 100 and 45 kyr BP in the Eastern Cantabrian region. In this way, we will be able to deepen our interpretation of the processes of cultural change of the last groups of Neanderthals that inhabited this region.

Holocene paleoceanographic variability in Robertson Bay, Ross Sea, Antarctica: A marine record of ocean, ice sheet, and climate connectivity

Accelerating ocean-driven basal melting of Antarctic ice shelves in recent decades has implications for sea level rise and global overturning circulation. Here, we reconstruct oceanographic conditions at the confluence of the Ross Sea and the Southern Ocean by analyzing a multi-proxy Holocene marine sedimentary record collected from Robertson Bay. A ramped pyrolysis oxidation radiocarbon age-depth model provides a timeline for glacial behavior and oceanographic changes over the last 6700 years. The diatom assemblage, magnetic susceptibility, grain size, total organic carbon and nitrogen, trace elements, and bulk δ13C are used as proxies for changing ocean and glacial conditions, which we interpret in the context of modern oceanographic measurements.Our record shows evidence of persistent ice cover in the northwestern Ross Sea during the Antarctic mid-Holocene climate optimum (ca. 5 cal kyr BP). Based on this observation, we suggest that meltwater and iceberg discharge associated with ice sheet retreat in the Ross Sea region altered local oceanography during the mid-Holocene. The onset of modern style oceanographic conditions in Robertson Bay occurred at ca. 4 cal kyr BP. Stable late Holocene conditions in are punctuated by a period of enhanced polynya activity and upwelling of nutrient rich Circumpolar Deep Water ca. 0.8 cal kyr BP and an increase in the seasonal duration of sea ice after 0.7 cal kyr BP, during the Little Ice Age. The response of the marine environment in Robertson Bay to mid-Holocene ice sheet retreat and natural climate variability during the last millennium underscores the sensitivity of the Antarctic ice-ocean interface to projected changes in coming decades.

Late Quaternary marine transgressions off the Shandong Peninsula inferred from paleosalinity indicators: Implications for Holocene mud wedge formation

Mud depocenters play an important role in the sediment source–sink system of marginal seas; however, the factors controlling the formation of mud deposits are not well understood. This study focused on Core LHSD-1, which is located on the southern margin of the Shandong Peninsula mud wedge in the northwestern part of the South Yellow Sea. Elemental ratios, such as the C/S ratio (the mass ratio of total organic carbon and total sulfur) and Sr/Ba ratio, were used to track the changes in paleosalinity during the late Quaternary and to investigate the mechanisms that influenced the formation of the mud wedge off the Shandong Peninsula. Our results indicate that Core LHSD-1 likely recorded two transgression events, corresponding to MIS3 and MIS1. Following sea-level rise after the Last Glacial Maximum (LGM), transgressive deposition began in the study area at approximately 11.5 kyr BP. During the early Holocene, fluctuating water salinity was recorded by Sr/Ba ratios, which became stable at approximately 6.8 kyr BP when the sea level reached its highstand. At the same time, the modern current system was established, and a significant amount of terrestrial material, such as refractory organic carbon and iron, has been transported to the study area by enhanced coastal currents, which was documented by elevated C/S ratios (C/S > 2.8) in mud sediments. Our findings indicate that marine transgressions controlled the initial development of the mud depocenter and that the establishment of the modern ocean current system promoted the rapid deposition of mud sediments, which corresponded to the growth of global mud depocenters.

A Highly Resolved Speleothem δ13C Record from Central China and Its Manifestation on Multiple Time Scales during the Last Glacial

Speleothem δ13C in monsoonal China differs from speleothem δ18O, which is widely used as a climatic proxy for several complex control reasons. Nevertheless, δ13C records have the potential to reveal the implications of hydroclimatic changes. This study reports a speleothem δ13C record from Didonghe (DDH) Cave in central China spanning 34 to 13 kyr BP. After we investigated the factors that influence speleothem δ13C, we found that the δ13C record showed that DDH Cave can prompt directional shifts via local hydroclimatic changes, such as in vegetation types, biomass, and rock–water interaction processes, suggesting that δ13C is mainly controlled by the local hydroclimate. Ensemble empirical mode decomposition (EEMD) results revealed a coupling relationship between δ18O and δ13C on multiple timescales, which suggested that changes in precipitation caused by large-scale monsoonal circulation are controlled by regional hydrological conditions to a great extent. However, the hydrological conditions of the cave were relatively mild and humid during the last glacial maximum (LGM), which revealed the impact of evaporation on changes in the region’s hydrological conditions. We also found that the δ18O and δ13C profiles decoupled when δ13C changed with a shift in the location of the westerly during HS1. The δ13C record correlates well with other paleoclimate records, suggesting that regional hydrological conditions are also modulated by the Earth’s internal and external driving factors.

Shore platform erosion and cliff retreat in the Eastern Korea: A quantified assessment using 10Be concentrations and numerical modeling

Shore platforms, essential for coastal analysis and management, are poorly understood in terms of their long-term evolution, particularly regarding coastal cliff retreat rates and trends, despite their common presence in rocky coastlines. Rock coasts constitute fully erosional environments, yet long-term rates and trends of coastal cliff retreat remain poorly understood. This study adds to the limited number of studies that use cosmogenic isotopes to reconstruct millennial-scale cliff erosion. Cosmogenic 10Be concentrations were measured in 16 rock samples collected across an active 31 m wide granitic Jangsa shore platform in eastern Korea. A geometry-based inverse numerical model was used to simulate 10Be concentrations for various cliff retreat modes, relative sea level curves, and platform downwearing models. The model results were combined with measured concentrations to find the most likely scenario for coastal evolution. Our findings reveal that the shore platform has widened through time and is entirely formed during the Holocene, unlike the previous cosmogenic study in western Korea that attributed the formation of shore platforms to former interglacial periods. The results suggest acceleration in cliff retreat rate from 1.4 mm yr−1 at 7.4 kyr BP to 7.0 mm yr−1 at present. Accelerating cliff erosion may pose a threat to coastal communities, particularly in the context of observed and predicted anthropogenic sea level rise.

Climate, vegetation, and fire, during the last deglaciation in northwestern Amazonia

The magnitude of change in climatic conditions and vegetation response to the last deglaciation in various parts of tropical Amazonia is poorly understood and controversial. Analysis of a sediment core e.g. fossil pollen, X-ray Fluorescence (XRF) and charcoal from Lake Malachite on the Hill of Six Lakes in northwestern Brazil provided a deglacial history of climate, vegetation change and fire. Pollen revealed a forested landscape throughout, with shifts in composition that were driven by warming and changes in precipitation. The glacial cooling of c. 4–5 °C had brought species characteristic of cooler climates into the Amazon lowlands and was followed by an initial warming that began at least 19.5 thousand calibrated years before the present (cal kyr BP). Temperature oscillations and changes in precipitation between (18–14.6 cal kyr BP) associated with Heinrich Stadial 1 were observed as wet-dry-wet oscillations similar to some of the previous studies, and were evident in both pollen and XRF data. The pollen spectra were consistent that of a mesic forest before and after the peak of the Last Glacial Maximum. Cool-adapted taxa had previously been described from other cores from the Hill of Six Lakes, and persisted in low abundances until c. 14.1 cal kyr BP. No distinct response to the Atlantic Cold Reversal was evident in our proxy data. The early Holocene was marked by pollen, charcoal, and sedimentary changes that could reflect a peak drought stress on the forest. The large occurrence of charcoal indicating an increase in fires coincided with disturbance elements e.g. Cecropia and Alchornea, that could have been consistent with human disturbance of the forest at c. 10.2 cal kyr BP.

Lake-level responses to climate change in an inland basin in the Japanese Islands during the last 16 kyr

During the last deglaciation and middle Holocene, rapid climatic changes occurred repeatedly. Previous research has primarily focused on investigating these periods in East Asia's coastal and inland regions of the Eurasian continent. In contrast, a limited number of studies have reported on climate change and its environmental impacts in island areas near the Pacific Ocean. This study aims to reconstruct paleosol drainage conditions and lake water level fluctuations in the Suwa Basin, an inland basin in central Japan, after the last deglaciation. We utilize the identification of paleosols and the geochemistry of lacustrine sediments to discuss climate change and its impacts on terrestrial environments in the East Asian coastal region. During the Last Glacial Maximum, the basin experienced a lowstand period; meandering fluvial systems led to the development of dry and well-drained paleosols. The lacustrine environment expanded between ca. 16 cal kyr BP and 5 cal kyr BP during a transgressive period. Short-term lake-level fluctuations were reconstructed by examining repeated paleosol development and changes in the bulk SiO2 content originating from biogenic silica in lacustrine sediments. Our data suggest that the lake's water level fell at ca. 12 cal kyr BP, 8 cal kyr BP, and 7 cal kyr BP. After 5 cal kyr BP, thin paleosols repeatedly formed during short exposure times due to the rapid sedimentation rate in the delta system, which corresponds to the highstand period. Short-term fluctuations between 16 kyr BP and 5 kyr BP, in the lake water level in the inland basin of the Japanese Islands was influenced by drying and cooling trends in the East Asian coastal region, as well as declines in sea surface temperatures in the western tropical Pacific Ocean. These fluctuations roughly synchronize with the Younger Dryas stadial and an 8.2 ka cooling event. Changes in precipitation, which responded to fluctuations in the East Asian summer monsoon intensity and tropical depressions, likely regulated the lake water level. These findings imply the predominant influence of paleoclimate fluctuations over thousands of years on the hydrological regime of lake systems across the island area of the East Asia.

Dynamical response of the southwestern Laurentide Ice Sheet to rapid Bølling–Allerød warming

Abstract. The shift in climate that occurred between the Last Glacial Maximum (LGM) and the Early Holocene (ca. 18–12 kyr BP) displayed rates of temperature increase similar to present-day warming trends. The most rapid recorded changes in temperature occurred during the abrupt climate oscillations known as the Bølling–Allerød interstadial (14.7–12.9 kyr BP) and the Younger Dryas stadial (12.9–11.7 kyr BP). Reconstructing ice sheet dynamics during these climate oscillations provides the opportunity to assess long-term ice sheet evolution in reaction to a rapidly changing climate. Here, we use glacial geomorphological inversion methods (flowsets) to reconstruct the ice flow dynamics and the marginal retreat pattern of the southwestern sector of the Laurentide Ice Sheet (SWLIS). We combine our reconstruction with a recently compiled regional deglaciation chronology to depict ice flow dynamics that encompass the time period from pre-LGM to the Early Holocene. Our reconstruction portrays three macroscale reorganizations in the orientation and dynamics of ice streaming followed by regional deglaciation associated with rapid warming during the Bølling–Allerød interstadial. Initial westward flow is documented, likely associated with an early set of ice streams that formed during the advance to the LGM. During the LGM ice streaming displays a dominant north to south orientation. Ice sheet thinning at ∼15 ka is associated with a macroscale reorganization in ice stream flow, with a complex of ice streams recording south-eastward flow. A second macroscale reorganization in ice flow is then observed at ∼14 ka, in which southwestern ice flow is restricted to the Hay, Peace, Athabasca, and Churchill river lowlands. Rates of ice sheet retreat then slowed considerably during the Younger Dryas stadial; at this time, the ice margin was situated north of the Canadian Shield boundary and ice flow continued to be sourced from the northeast. Resulting from these changes in ice sheet dynamics, we recognize a three-part pattern of deglacial landform zonation within the SWLIS characterized by active ice margin recession, stagnation and downwasting punctuated by local surging (terrestrial ice sheet collapse): the outer deglacial zone contains large recessional moraines aligned with the direction of active ice margin retreat; the intermediate deglacial zone contains large regions of hummocky and stagnation terrain, in some areas crosscut by the signature of local surges, reflecting punctuated stagnation and downwasting; and the inner deglacial zone contains inset recessional moraines demarcating progressive regional ice margin retreat. We attribute these macroscale changes in ice flow geometry and associated deglacial behaviour to external climatic controls during the Bølling–Allerød and Younger Dryas but also recognize the role of internal (glaciological, lithological and topographic) controls in SWLIS dynamics.

A Greenland-wide empirical reconstruction of paleo ice sheet retreat informed by ice extent markers: PaleoGrIS version 1.0

Abstract. The Greenland Ice Sheet is a large contributor to global sea level rise, and current mass losses are projected to accelerate. However, model projections of future ice sheet evolution are limited by the fact that the ice sheet is not in equilibrium with present-day climate but is still adjusting to past changes that occurred over thousands of years. While the influence of such committed adjustments on future ice sheet evolution remains unquantified, it could be addressed by calibrating numerical ice sheet models over larger timescales and, importantly, against empirical data on ice margin positions. To enable such paleo data–model interactions, we need Greenland-wide empirical reconstructions of past ice sheet extent that combine geomorphological and geochronological evidence. Despite an increasing number of field studies producing new chronologies, such a reconstruction is currently lacking in Greenland. Furthermore, a time slice reconstruction can help to (i) answer open questions regarding the rate and pattern of ice margin evolution in Greenland since the glacial maximum, (ii) develop a standardised record of empirical data, and (iii) identify new sites for future field campaigns. Based on these motivations, we here present PaleoGrIS 1.0, a new Greenland-wide isochrone reconstruction of ice sheet extent evolution through the Late Glacial and early- to mid-Holocene informed by both geomorphological and geochronological markers. Our isochrones have a temporal resolution of 500 years and span ∼ 7.5 kyr from approximately 14 to 6.5 kyr BP. We describe the resulting reconstruction of the shrinking ice sheet and conduct a series of ice-sheet-wide and regional analyses to quantify retreat rates, areal extent change, and their variability across space and time. During the Late Glacial and early- to mid-Holocene, we find the Greenland Ice Sheet has lost about one-third of its areal extent (0.89 million km2). Between ∼ 14 and ∼ 8.5 kyr BP, it experienced a near-constant rate of areal extent loss of 170 ± 27 km2 yr−1. We find that the ice-sheet-scale pattern of margin retreat is well correlated to atmospheric and oceanic temperature variations, which implies a high sensitivity of the ice sheet to deglacial warming. However, during the Holocene, we observe inertia in the ice sheet system that likely caused a centennial- to millennial-scale time lag in ice extent response. At the regional scale, we observe highly heterogeneous deglacial responses in ice extent evident in both the magnitude and rate of retreat. We hypothesise that non-climatic factors, such as the asymmetrical nature of continental shelves and onshore bed topographies, play important roles in determining the regional- to valley-scale dynamics. PaleoGrIS 1.0 is an open-access database designed to be used by both the empirical and numerical modelling communities. It should prove a useful basis for improved future versions of the reconstruction when new geomorphological and geochronological data become available.

Holocene wildfire regime shifts induced by the enhancement of human activities in the Changjiang (Yangtze River) Basin

Deforestation and technical innovation have changed the natural fire regimes, topography and carbon reservoirs during the history of human evolution. Recognition of the timing, scope, and magnitude of wildfire events in large river basins such as the Changjiang (Yangtze River) is essential for understanding the impacts of climate forcing and human perturbation on Earth surface processes. Here, we present a multi-proxy study based on the pollen and black carbon (BC) abundance in sediments of Core CM97 taken from the Changjiang Delta, aiming to reconstruct the paleo-fire history in the Changjiang River Basin and further disentangle anthropogenic and climatic influences on fire regimes during the Holocene. Our results show that the average BC abundance increased from 0.88 ‰ at 12–8 cal kyr BP to 1.17 ‰ at 8–5 cal kyr BP, well corresponding to the evolution of the Asian Summer Monsoon (ASM). This suggests the wildfire occurrences were dominated by hydroclimatic conditions during 12–5 cal kyr BP. After the early Bronze Age (∼5 cal kyr BP), the sustained high anomaly of BC abundance (average 1.30 ‰), however, was decoupled from the weakened monsoon climate. We propose that this might be associated with anthropogenic destruction of natural forests through fire-assisted agriculture and bronze smelting. Especially, with the prosperity of the Han Dynasty (202 BCE − 220 CE), a rapid increase in population promoted the transition of ecosystem (vegetation type) from forests to permanent farmlands in the drainage basin. Human intervention has overwhelmed the long-term climate control over wildfire, plant diversity, soil erosion, and sediment source-to-sink processes, suggesting the dominance of anthropogenic activities on nature over the last 2 kyrs. Our study provides deep insight on the enhancing perturbation of human activities on Earth's surface at a continental scale and provides more constraints on the Anthropocene epoch.

Coccolithophore and paleoproductivity variations in the subtropical western South Atlantic since the late deglaciation

Here we present a record of coccolith variation over the past 14 kyr BP from core KF-02 (25.84°S, 45.2°W; 827 m water depth) collected from the continental slope of the São Paulo Bight (SPB), located in the subtropical western South Atlantic. Additionally, we estimated primary productivity based on Florisphaera profunda nannoliths and compared it with other published investigations on the productivity based on different proxies. The coccolith assemblages varied and agreed with the main regional oceanic and atmospheric changes since the latest deglaciation, likely in response to the terrestrial influence and nutricline depth variations. Among eutrophic species, Gephyrocapsa oceanica was more abundant during times with enhanced productivity promoted by the input of terrestrial material. Emiliania huxleyi and small Gephyrocapsa dominated the middle Holocene samples and probably benefitted from the more stable oceanic conditions and the enhanced light availability promoted by the Brazil Current. The paleoproductivity estimates indicated that moderate levels of productivity persisted since the latest deglaciation with a decreasing trend after the middle Holocene, delaying the productivity reduction that was indicated by other regional studies based on planktonic foraminifera. Apparently, seasonal regional hydrographic conditions were responsible for changing coccolith assemblages, and these were not reflected by the planktonic foraminifera G. bulloides variations.

Middle to late Holocene hydroclimate variability revealed by multi-proxy stalagmite records from Jinfo Cave, central-west China

The pattern and cause of middle-to late-Holocene hydroclimatic variability in central-west China remain unclear. We present a new pair of stalagmite isotopic (δ18O and δ13C) records and trace element ratio (Mg/Ca, Ba/Ca, and Sr/Ca) records from Jinfo Cave, central-west China. We established a uranium-series chronology that shows that the stalagmite grew between 5.6 and 0.6 kyr BP. The similarities between our δ18O stalagmite record and other published records suggest that δ18O generally reflects overall monsoon intensity or rained-out effect between the cave site and moisture source, but not necessarily regional rainfall amount. Corresponding changes in Mg/Ca, Ba/Ca, Sr/Ca, and δ13C are mostly dependent on local factors, such as the nature of the ecosystem above the cave, water-rock interactions, and the prior calcite precipitation effect. As the δ13C records bear a remarkable resemblance to Mg/Ca, Ba/Ca, and Sr/Ca records, we extracted the first principal component (PC1) of the four records and identified the PC1 as a regional hydroclimate proxy. The bandpass filter and cross wavelet transformation results show that the ∼500 years periodicity of PC1 exhibits in-phase oscillations with solar activity and the El Niño-Southern Oscillation (ENSO) throughout the middle to late Holocene, suggesting that changes in solar activity influence centennial hydroclimate variation and are amplified by ENSO. Another prominent feature seen in PC1 is a progressive positive shift from 3.0 kyr BP, which indicates that central-west China moved toward a drier climate over this period. We attribute this climate transition to a strengthening El Niño event and its impact on oceanic and atmospheric anomalies.