Ka Period Research Articles

Fluvial landscape change in Anqing through the last glacial cycle: Implications for eustatic controls on the Yangtze River's continental-scale incision-aggradation cycles

The Yangtze River is known as the ‘mother river of China’ because it flows through the country's most densely populated and historically wealthiest regions. However, the evolution of the Yangtze River and its controls remain unclear. In this study we dated samples extracted from five bedrock-penetrated boreholes in Anqing, in the lower Yangtze River Valley. The resultant optically stimulated luminescence (OSL) and radiocarbon (14C) dates would suggest that fluvial sediments accumulated during the ∼33–44 ka and ∼11-0.2 ka periods, with extensive erosion during the >44 ka and 33-11 ka timeframes. We also reviewed data from 13 OSL and 14C dated boreholes located between the middle reaches of the Yangtze River and its estuary. The results would suggest that most of the fluvial sediments were deposited during Marine Isotope Stages (MIS) 3 and 1, with a clear erosion hiatus during the Last Glacial Maximum (LGM). Glacial-interglacial cycle-driven eustasy was the main driver of the Yangtze River's incision-aggradation process. The eustatic forcing during the glacial-interglacial cycle influenced the changes in the landscape of the Yangtze River Valley as far inland as ∼1200 km from the present-day shoreline.

Records of trace metals since the Holocene in sediments of Laizhou Bay, Bohai Sea, China

Trace metal (Cu, Pb, Zn, Cr, Cd, As, Hg, Ni, Li, and Co) concentration profiles along a sediment core were investigated to obtain background values, assess depositional trends and contamination levels in the Laizhou Bay, China. The enrichment factors of the metals indicated no anthropogenic enrichment, except for Pb, Cd, and Hg, which have been influenced by human activities since 7 ka. The mean concentrations of trace metals in the 10–7 ka phase can be considered as the background values in the study area. Principal component analysis indicated that the metals were mainly naturally sourced throughout the observed depositional period. Their geoaccumulation indices indicated that the sediments were not contaminated by Cu, Zn, Cr, Ni, and Co, but were contaminated by Pb, Cd, and Hg during the 7–0 ka period.

Multi‐Centennial Variability of Yangtze Delta Growth Over the Last 2000 Years: Interplay of Climate and People

AbstractReconstruction of sediment accumulation in river deltas over the Holocene provides a basis for understanding the relationship between climate change, human activities and delta growth. However, variations in deposition rates on a centennial‐scale over the last 2000 years remain poorly studied for mega‐deltas. Based on optically stimulated luminescence and AMS 14C ages, we calculated deposited sediment volumes in the Yangtze River Delta (YRD) for 500‐year intervals over the last two millennia for the first time. Our results reveal that the 2.0–1.5 and 0.5–0 ka periods had higher sediment deposition rates than in 1.5–1.0 and 1.0–0.5 ka, with maximum value about two times that of the minimum. A comparison with historical records of flooding and drought events indicates that a wetter climate led to a higher sediment deposition, reflecting the role of fluvial discharge in delta growth. However, the highest sediment deposition (205 ± 29 × 106 m3/a) over the last 500 years is not only related to the wetter conditions of the Little Ice Age, but also linked to dramatic regional population growth. We suggest that human activities have predominated over natural forcing in determining the deltaic growth over the last five centuries. Taking the Medieval Climate Anomaly with the lowest deposition volume as an analog of current climate warming, and considering the decline in sediment loads due to recent damming and soil conservation, the YRD is likely to face an even more severe deficit in sediment supply and higher risk of delta destruction in the coming centuries.

Inverse Magnetic Fabrics Caused by Magnetofossils in the Northwestern South China Sea Since End of the Last Glacial

AbstractThe relationships among the abundance of magnetofossils, the ensuing magnetic properties, and the controlling paleoenvironmental factors in marine sediments remain broadly unexplored. Here, we identify magnetofossils in core XB1 from the northwestern South China Sea (SCS) since end of the Last Glacial. Using rock magnetic and electron microscopic data, we propose a model that links the anisotropy of magnetic susceptibility fabric and the abundance of magnetofossils. The magnetofossil concentration in sediments increases significantly during the 14.7–4.7 ka period, which in turns leads to inverse magnetic fabrics and near‐horizontal of the minimum magnetic susceptibility axes. Further, we show that the abundance of magnetofossils is linked to paleoenvironmental changes in the northwestern SCS. The production and preservation of magnetofossils during the 14.7–4.7 ka period are promoted by an intensified East Asian summer monsoon and sluggish deep‐water ventilation, while the paucity of magnetofossils after 4.7 ka is attributed to high oxygen content.

Spatiotemporal Discharge Variability of the Doce River in SE Brazil During MIS 6 and 5

The modern precipitation balance in southeastern (SE) Brazil is regulated by the South American summer Monsoon and threatened by global climate change. On glacial-interglacial timescales, monsoon intensity was strongly controlled by precession-forced changes in insolation. To date, relatively little is known about the spatiotemporal distribution of tropical precipitation in SE Brazil and the resulting variability of fluvial discharge on glacial-interglacial timescales. Here, we present X-ray diffraction-derived mineralogical data for the 150–70 ka period (marine isotope stage (MIS) 6 to MIS 5) from the Doce River basin. This area was sensitive to changes in monsoonal precipitation intensity due to its proximity to the South Atlantic Convergence Zone. The data, obtained from a marine sediment core (M125-55–7) close to the Doce river mouth (20°S), show pronounced changes in the Doce River suspension load’s mineralogical composition on glacial-interglacial and precessional timescales. While the ratio of silicates to carbonates displays precession-paced changes, the mineralogical composition of the carbonate-free fraction discriminates between two assemblages which strongly vary between glacial and interglacial time scales, with precession-forced variability only visible in MIS 5. The first assemblage, dominated by high contents of kaolinite and gibbsite, indicates intensified lowland erosion of mature tropical soils. The second one, characterized by higher contents of the well-ordered illite, quartz and albite, points to intensified erosion of immature soils in the upper Doce Basin. High kaolinite contents in the silicate fraction prevailed in late MIS 6 and indicate pronounced lowland soil erosion along a steepened topographic gradient. The illite-rich mineral assemblage was more abundant in MIS 5, particularly during times of high austral summer insolation, indicating strong monsoonal rainfall and intense physical erosion in the upper catchment. When the summer monsoon weakened in times of lower insolation, the mineral assemblage was dominated by kaolinite again, indicative of lower precipitation and runoff in the upper catchment and dominant lowland erosion.

Calcium Carbonate Dissolution Triggered by High Productivity During the Last Glacial–Interglacial Interval in the Deep Western South Atlantic

Studies reconstructing surface paleoproductivity and benthic environmental conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the seafloor, we studied the sediment core SAT-048A, spanning 43–5 ka, recovered from the continental slope (1,542 m water depth) of the southernmost Brazilian continental margin, deep western South Atlantic. We assessed the sea surface productivity, the organic matter flux to the seafloor, and calcite dissolution effects, based on micropaleontological (benthic and planktonic foraminifers, ostracods), geochemical (benthic δ13C isotopes), and sedimentological data (carbonate and bulk sand content). Superimposed on the induced changes related to the last glacial–interglacial transition, the reconstruction indicates a significant and positive correlation between the paleoproductivity proxies and the summer insolation. From the reconstructed data, it was possible to identify high (low) surface productivity, high (low) organic matter flux to the seafloor, and high (low) dissolution rates of planktonic Foraminifera tests during the glacial (postglacial). Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, explained by increased northeasterly summer winds that promoted meandering and upwelling of the nutrient-rich South Atlantic Central Water. Statistical analyses support the idea that productivity is the main cause for seafloor calcium carbonate dissolution, as opposed to changes in the Atlantic Meridional Overturning Circulation (at least for the 25–4 ka period). Further efforts must be invested in the comprehension and quantification of the total organic matter and biogenic carbonate burial during time intervals with an enhanced biological pump, aiming to better understand their individual roles.

Geomorphology and 10Be chronology of the Last Glacial Maximum and deglaciation in northeastern Patagonia, 43°S-71°W

In southern South America, well-dated glacial geomorphological records constrain the last glacial cycle across much of the former Patagonian Ice Sheet, but its northeastern sector remains comparatively understudied and unconstrained. This knowledge gap inhibits our understanding of the timing of maximum glacier extent, the duration of the glacial maximum, the onset of deglaciation, and whether asynchronies exist in the behaviour of the former ice sheet with latitude, or with location (east or west) relative to the ice divide. Robust glacial reconstructions from this region are thus required to comprehend the mechanisms driving Quaternary glaciations at the southern mid-latitudes. We here present 10Be surface exposure ages from five moraine sets along with Bayesian age modelling to reconstruct a detailed chronology of Last Glacial Maximum expansions of the Río Corcovado glacier, a major former ice conduit of northern Patagonia. We find that the outlet glacier reached maximum expansion of the last glacial cycle during the global Last Glacial Maximum at ∼26.5–26 ka, and that at least four subsequent advances/stillstands occurred over a 2–3 ka period, at ∼22.5–22 ka, ∼22–21.5 ka, ∼21–20.5 ka and 20–19.5 ka. The onset of local ice sheet deglaciation likely occurred between 20 and 19 ka. Contrary to several other Patagonian outlet glaciers, including from similar latitudes on the western side of the Andes, we find no evidence for MIS 3/4 advances. Exposure dating of palaeo-shoreline cobbles reconstructing the timing of proglacial lake formation and drainage shifts in the studied region indicate three glaciolacustrine phases characterised by Atlantic-directed drainage. Phase one occurred from 26.4 ± 1.4 ka, phase two between ∼21 and ∼19 ka and phase three between ∼19 ka and ∼16.3 ka. Exposure dating of ice-moulded bedrock in the interior of the cordillera indicates local disintegration of the Patagonian Ice Sheet and the Atlantic-Pacific drainage reversal had occurred by ∼16.3 ka. We find that local Last Glacial Maximum glacier expansions were coeval with Antarctic and southern mid-latitude atmospheric and oceanic cooling signals, but out of phase with local summer insolation intensity. Our results indicate that local Patagonian Ice Sheet deglaciation occurred 1–2 ka earlier than northwestern, central eastern and southeastern Patagonian outlet glaciers, which could indicate high regional Patagonian Ice Sheet sensitivity to warming and drying during the Varas interstade (∼22.5–19.5 ka).

Meridional migration of Indian Ocean Monsoon precipitation during the early Holocene: Evidence from the Andaman Sea

The Holocene evolution of the Indian Ocean Monsoon (IOM), especially the spatiotemporal nature of its maximum precipitation period, remains controversial. Here we reconstruct high-resolution local δ18Osw and sea surface temperature (SST) values by pairing Mg/Ca values and the δ18O data from planktonic foraminifera Globigerinoides ruber (white) in the southern Andaman Sea in order to understand IOM rainfall variability during the Holocene. Local δ18Osw values show that local maximum rainfall occurred between 10.6 ka and 8.6 ka in the southern Andaman Sea, before declining markedly during the 8.6–5.0 ka period, and then decreasing slightly after 5.0 ka. The Mg/Ca-SST was relatively low during the 10.3–8.6 ka period, before increasing slightly between 8.6 ka and 4.0 ka; it declined thereafter. But the sedimentation rate at the site of Core ADM-C1 relating to monsoonal precipitation of the source region of sediments exhibited its highest value during the 8.6–5.5 ka period. After making further comparisons with other published records for this region, we would propose that maximum IOM precipitation during the early Holocene has exhibited a clearly meridional migration: it was located in the tropics around 7°N during the 10.6–8.6 ka period, migrated northward to the subtropics during the 8.6–5.5 ka period, before migrating southward again after 5.0 ka. We would suggest that the northward shift of maximum precipitation during the Early Holocene in the IOM region was in response to the progressively northward migration of the Intertropical Convergence Zone (ITCZ) and the change of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) activities.

Late Quaternary variations in paleoerosion rates in the northern Qilian Shan revealed by 10Be in fluvial terraces

Understanding the pace of erosion through space and time may provide insight into how tectonic and climatic processes conspire to drive landscape evolution. However, determining whether erosion rates in a given locality have varied with past climate change remains challenging, and the mechanisms invoked to explain variations in past erosion rates are not well understood. In this study, we evaluate past rates of erosion during the late Quaternary along the northern Qilian Shan, the range bounding the northeastern margin of the Tibetan Plateau. We measured the concentrations of cosmogenic 10Be from fluvial deposits atop 5 terraces along the Hongshuiba River and average 10Be concentrations at the time of terrace formation. We also determined terrace ages by combining 5 10Be depth profiles and 15 OSL samples of overlying loess. Our results confirm previous dating of this terrace sequence using cosmogenic exposure ages of boulders and depth profiles in terrace gravels and indicate that terrace treads span glacial-interglacial cycles over the past ~200 ka. We exploit this exceptionally dense sampling of 10Be to evaluate differences in cosmogenic 10Be concentrations at the time of terrace formation along the Hongshuiba River. Analysis of 48 individual determinations of “inherited” 10Be concentrations reveal that fluvial sediment in terraces abandoned during the last glacial and penultimate glacial periods are 3–4 times greater than concentrations in sediment atop terraces formed during interglacial times. Paleoerosion rates within the Hongshuiba watershed appear to have been ~200 m/Myr during glacial episodes but increased to ~500–700 m/Myr during interglacial times, similar to recent erosion rates inferred from 10Be in modern sediment. These results point to likely changes in the mean residence time of sediment with the watershed that fluctuate in association with 100-ka glacial-interglacial cycles. We suggest that reductions in erosion rates associated with glacial conditions may have been driven by changes in catchment hydrology during cold and dry periods.

Orbitally driven evolution of Asian monsoon and stable water isotope ratios during the Holocene: Isotope-enabled climate model simulations and proxy data comparisons

The variation of stable water isotopes (δ18O, δD) in climate archives is an important proxy to understand the evolution of South Asian monsoon (SA) precipitation over the Holocene. In this study, using an isotope-enabled climate-model, we examine the responses of water isotopes in precipitation over the SA region to orbital changes in the early to late Holocene (8 ka to 0 ka). Precipitation is enhanced during 8 ka to 4 ka in response to increased summertime insolation, and correspondingly, larger magnitudes of negative water isotope ratios in precipitation (δ18Oprecip) are simulated. The model-simulated wettest period and the corresponding period of maximum depletion of δ18Oprecip in the SA region is the 8 ka period. We find that strengthened circulation, increased convection, and precipitation led to the δ18Oprecip depletion in the SA region. In the tropical Indian monsoon region, where convective precipitation is dominant, the δ18Oprecip values are inversely correlated with the local convection and the amount of precipitation. The δ18Oprecip values in the East Asian Summer Monsoon (EASM) region are not well-correlated with local precipitation, likely due to the enhanced convection and depletion of vapor in upstream areas and mixed precipitation types in the region. The modeled δ18Oprecip values are evaluated against δ18O inferred from proxy archives. Proxy data and model-simulated isotope ratios in precipitation agree on the larger magnitude of negative isotopic values during the 4 ka, 6 ka, and 8 ka periods relative to the present, although model-simulated enrichment in the 2 ka period is inconsistent with some proxy-records.

Dam and megafloods at the First Bend of the Yangtze River since the Last Glacial Maximum

The Yangtze River is the longest watercourse in China, where it flows through the most densely populated and richest regions. The upper course of the River (Jinsha River) has been recurrently blocked over the course of time. The dam-outburst flood process has exerted a significant impact on the river's drainage area. However, the frequency of such events along the course of the Yangtze River remains poorly investigated. In this study, damming chain sediments were identified in the First Bend area of the Yangtze River, and were dated using optically stimulated luminescence (OSL) and 14C techniques. The obtained ages would suggest that the river was dammed by a debris flow during the ~20–7 ka period, with a resulting paleolake covering an area of 40.79 km2 and having a water volume of 0.63 km3. Two outburst megafloods occurred at ~18–20 and ~ 6.0 ka, respectively. The river was re-dammed again during 1.5–0.4 ka, with a lake coverage of 28.20 km2 and water volume of 0.28 km3. The dammed lake may have been gradually breached after ~0.4 ka. The damming events were caused by debris flows triggered by glacial advances on Yulong Mountain.

Assessing the climatically triggered aggradation-incision processes in a dryland environment during the Late Quaternary period from Shetrunji River basin (Saurashtra), India

The present study is an attempt to infer the response of the dryland fluvial sequences to paleoenvironmental change since the last glacial maximum (LGM) from the semi-arid Saurashtra region, a basaltic terrain of the Late Cretaceous age (Deccan Trap), of Western India. We employed a conventional sedimentological analysis supported by sediment geochemistry, which was constrained by optical dating. Based on detailed field investigation, we identified five terrace sequences from the valley fill deposit in the Shetrunji River basin since 18 ka (i.e. Post LGM). The sedimentary archives expose the deposition during three alternate phases of aggradation and incision, marked on the basis of morphostratigraphy and timing of landform evolution. The coastal region of western India experienced a widespread valley aggradation in response to the rising sea stand and under gradual strengthening of the monsoonal conditions around the 18–9 ka period. The Early Holocene climatic optima (9–7.5 ka) coupled with lower than present but rising sea stand, triggered the intense erosion in the valleys of Shetrunji River basin, leading to erosion/lack of sediment preservation. This was followed by the Middle Holocene period (7.5–5 ka), which experienced valley aggradation. This was ended by the incision post 5.5 ka in the lower reaches of the Shetrunji River. The sea stand was similar to present-day stand, as evidenced by the change from fluvial to the estuarine environment in the top sequence of Site-SL at 5.5 ka. The Late Holocene relative sea stand lowering ~3 ka, led to an incision in the estuarine terrace of about 2 m. The causal mechanism for triggering of this incision is most likely the relative fall in sea stand to present-day level from its Middle Holocene high stand. The Late Holocene period (3–1 ka) shows aggradation in upper reach in fluctuating hydrodynamic conditions with seasonality, followed by a relatively good monsoonal condition at < 1 ka, leading to the incision due to climate-tectonic interplay. These pulses of sedimentation could be recognized and explored as aided by basin-scale approach as in the present study. Site-specific studies do not enjoy such liberty.

Wildfire evolution and response to climate change in the Yinchuan Basin during the past 1.5 Ma based on the charcoal records of the PL02 core

Wildfire activity is an important part of natural ecosystems and is strongly influenced by climate and vegetation changes. Given this, the reconstruction of wildfire history is crucial to understanding the interrelationships between past climate change and fire dynamics, especially in terms of revealing past climate change characteristics during particularly important periods. In this study we employed a macro-charcoal sequence to reconstruct the wildfire history in the Yinchuan Basin (an arid and semi-arid region in China) during the past 1.5 Ma. The responses of wildfire activity to climate and vegetation changes were compared with other obtained environmental indices such as magnetic susceptibility, pollen in the terrestrial basin core (the PL02 core), and δ18O in the marine core (LR04). The results confirmed that macro-charcoal records are an effective and sensitive indicator of environmental evolution in arid and semi-arid areas. The peak values of charcoal concentration and charcoal flux generally occurred during climate transition periods, especially in the transition period from humid to drought conditions (specifically in the MIS41-MIS40, MIS39-MIS38, MIS19-MIS18, MIS17-MIS16, MIS15-MIS14, MIS11-MIS10, MIS9-MIS8, and MIS3-MIS2 time intervals). Furthermore, we found that the biomass burned in wildfires (i.e., the high biomass burned as indicated by the peak levels of charcoal concentration) was controlled by the amount of vegetation, while the occurrence of wildfires was related to climate fluctuations. In addition, some Milankovitch cycles (such as 100-ka cycles, 40-ka cycles, and 20-ka cycles) were evident in the charcoal records, thus indicating that the main driving force of wildfire activity in the Yinchuan Basin was climate change caused by variations in the Earth’s orbit. Among these, the 40-ka cycles disappeared after 0.72 Ma, accompanied by changes in wildfire activity characteristics. We also discovered that the 400-ka cycles were stable throughout time. This study provides reliable evidence for some remarkable climate events, including the Mid-Pleistocene Transition (MPT) and the Mid-Brunhes Event (MBE), from the charcoal records in the Yinchuan Basin (an arid and semi-arid region in the mid-latitudes of the Northern Hemisphere).

Orbital and sea-level changes regulate the iron-associated sediment supplies from Papua New Guinea to the equatorial Pacific

Iron (Fe) supplied to the equatorial Pacific are thought to play a critical role in marine biogeochemical cycle and the related air–sea CO2 flux. Previous studies have shown that Papua New Guinea (PNG) is a major Fe contributor for the western equatorial Pacific. However, key issues of this Fe supply including the exact sources, transport processes and distribution range remain unclear, especially its long-term variability associated with hydroclimatic and paleoceanographic changes. Using a set of cores collected offshore the northern PNG and from the Ontong-Java Plateau (OJP), shown to situate along the pathway of Fe delivery by undercurrents, here we compile high-resolution records to explore how the Fe-associated sediment supplies varied and distributed on orbital time-scale during the past 400 ka. The chronologic frameworks of these cores are established by using 14C datings and stratigraphic correlations of foraminiferal δ18O records and carbonate contents. The sedimentary Fe and clay-mineral smectite in the offshore-PNG sites show a prominent precession cycle, in phase with the March insolation change at 5°S. Such precessional predominance suggests the control of local precipitation and river runoff on the offshore deposition of the Fe-associated sediment. For the OJP sites the proxies derived from terrigenous detrital component give a spectral pattern, however, prevailed by ∼100-ka cycle with secondary precessional highs. The significant 100-ka cycle can best be attributed to glacial–interglacial changes in oceanic transports, likely caused by sea-level regulated variability in the strength and/or route of the coastal undercurrents off PNG. This variability may have influenced the long-distance transport of Fe-rich particulates from the PNG to the OJP, and further eastwards via the Equatorial Undercurrent.

Holocene Critical Zone dynamics in an Alpine catchment inferred from a speleothem multiproxy record: disentangling climate and human influences

Disentangling the effects of climate and human impact on the long-term evolution of the Earth Critical Zone is crucial to understand the array of its potential responses to the ongoing Global Change. This task requires natural archives from which local information about soil and vegetation can be linked directly to climate parameters. Here we present a high-resolution, well-dated, speleothem multiproxy record from the SW Italian Alps, spanning the last ~10,000 years of the present interglacial (Holocene). We correlate magnetic properties and the carbon stable isotope ratio to soil stability and pedogenesis, whereas the oxygen isotope composition is interpreted as primarily related to precipitation amount, modulated at different timescales by changes in precipitation source and seasonality. During the 9.7-2.8 ka period, when anthropic pressure over the catchment was scarce, intervals of enhanced soil erosion are related to climate-driven vegetation contractions and occurred during drier periods. Immediately following the onset of the Iron Age (ca. 2.8 ka), by contrast, periods of enhanced soil erosion coincided with a wetter climate. We propose that the observed changes in the soil response to climate forcing were related to early anthropogenic manipulations of Earth’s surface, which made the ECZ more sensitive to climate oscillations.

Multiscale analysis of Asian Monsoon over the past 640 ka

The empirical mode decomposition (EMD) method is used to re-analyse the high-resolution and precisely-dated stalagmite record from Chinese caves over the past 640 ka. Results show that (1) the variation in the Asian Monsoon can be completely decomposed into ten quasiperiod oscillations, among which the precession and semiprecession band oscillations are the most prominent periodicities, with contribution rates of 31.1% and 30.7%, respectively; (2) the cross-spectrum analysis of the semiprecession component and bi-hemisphere insolation (BHI) are strongly correlated, indicating an amplified response of precipitation and temperature variability to the interhemispheric insolation in the low-latitude regions, thus further affecting the intensity of the Asian Monsoon; (3) on millennial timescales, obvious oscillations at the 5 ka and 1–2 ka bands roughly correspond to the classical Bond and Dansgaard-Oeschger (DO) cycles. Additionally, a strong correlation is found between the detrended stalagmite δ18O records and Ca/Sr sequence from the North Atlantic (especially at the 5 ka band). This result means that the 5 ka cycle is characteristic of the glacial-interglacial cycle since the middle and late Pleistocene and may imply that climate change on the millennial timescale is the result of an interaction between global ice volume and insolation.

A 14‐ka Record of Dust Input and Phytoplankton Regime Changes in the Subtropical NE Pacific: Oceanic and Terrestrial Processes Linked by Teleconnections at Suborbital Scales

AbstractWe study shifts in phytoplankton proxies linked to terrigenous inputs and teleconnections in a core from Soledad Basin, Gulf of Ulloa, NW Mexico, spanning the end of the deglaciation and the Holocene. We used biogenic opal (% opal), organic carbon (% total organic carbon [TOC]), and inorganic carbon (% CaCO3) as proxies of productivity and opal/TOC and CaCO3/TOC ratios as proxies of nutrient uptake and C‐export by siliceous and carbonate organisms. We reconstructed terrestrial inputs and identified authigenic gypsum. Based on opal/TOC and CaCO3/TOC ratios, we found periodic changes of ~0.5, 1.1–1.8 ka cycle in phytoplankton proxies exporting siliceous and carbonate skeletal debris to the sediments. An increase in carbonate organisms occurred during 14–8.7 ka, corresponding to reduced El Niño–Southern Oscillation (ENSO)‐like variability, in parallel to the northward displacement of the Intertropical Convergence Zone (ITCZ) and an overall negative phase of the Pacific Decadal Oscillation (PDO). An increase in siliceous organisms occurred between 6 and 3 ka, coincident with strong ENSO‐like conditions, southern migration of ITCZ, and less intense but more frequent positive PDO‐hydrological variability. Grain size analyses show significant amounts of fine fraction (dust &lt;6.6 μm) present during the early‐ to mid‐Holocene in agreement to extreme weather on land, with episodes of eolian and fluvial transport to the sea. The ENSO‐like variations influenced biological C‐export producers on a scale of 1.1–1.8 ka, but PDO‐related variability is uncertain. We suggest that Holocene drivers for phytoplankton successions are changes in insolation, ITCZ migration, California Current upwelling, nutrient inputs by advection, and terrestrial sources.

Late Holocene palaeo-environmental change in the Banni Plains, Kachchh, Western India

The effect of the environmental change during the Middle to Late Holocene period, on the demise of the mighty Harappan civilization has remained a topic of debate. The western India, especially the coastal regions, hosted a cluster of Harappan settlements near the shoreline. The present study attempts to reconstruct the evolution of Banni Plains of the Kachchh region, which has remained a hotspot for Harappan civilization. We employed a host of multi-proxy techniques viz., sedimentology, sediment geochemistry, magnetic susceptibility along with OSL chronology, to deduce the palaeo-environmental changes experienced by the Banni Plains from a shallow 4.05 m deep trench. The four OSL chronology of the sediments suggests the time span of 2.9 ka to 0.2 ka, revealing a first hand estimate of average sedimentation rate of 1.5 mm per annum. The enhanced chemical weathering intensities suggest that the last two millennia witnessed two phases of relatively warm and humid climatic conditions, which probably coincide with the Roman Warm Period (?) and the Medieval Warm Period (?). The Banni Plains was evolved owing to the Middle Holocene high sea stand and subsequent relative fall during the last 2 ka period. The extent of the relative high sea stand upto 2 m above msl also corroborated well by ancient port settlements of the Late Harappan period. This implies that the Harappan civilization was indeed influenced and affected by the past sea level change; nevertheless, this cannot be the sole reason for demise of the mighty civilization, but remained an auxiliary mechanism along with aridity which severed the human adaptation.

Style and stages of valley fill aggradation-incision cycles in the Northern Hill Range, Kachchh, Western India

Sedimentary sequences often provides ample of clues to reconstruct the past climatic as well as tectonic perturbations undergone by a region, however owing to similar residual products the differentiation is an intricate issue. We report the aggradation and incision phases along the Northern Hill Range (NHR), Kachchh; one of the most seismically active, but poorly explored, intraplate environments on earth. The northerly draining fluvial sequences in the NHR, exhibit a variety of valley fill, channel fill and strath terrace sequences over the hanging wall block of one the most seismically active reverse faults in the world ∼ Kachchh Mainland Fault (KMF). The study of these sequences owing to its position, enables us to reconstruct the uplift history of the KMF as well as scrutinize the signatures of climatic and tectonic events in sedimentation since the last glacial period. Based on detailed stratigraphy and OSL chronology, we identified five cyclic phases of aggradation and incision during the last 24 ka period. The NHR experienced two aggradation phases during 24–10 ka and 8–4 ka period in response to climate change. Simultaneously, the KMF experienced at least two phases of uplift 1) > 24 ka and <4 ka, which led to landscape modification at regional scale inform of terrace formation. Global events such as Early Holocene Optima and 4.2 ka arid phase are also archived in form of strong erosional phase in the fluvial sequences of NHR. Although due to paucity of archives and discreet nature of sedimentary succession in dryland environments, the actual causal mechanism for the incision at 4 ka period remains ambiguous. The longterm minimum average uplift rate of the eastern KMF varies from 0.1 to 1.0 mm/a, which is directly linked to the longterm deformation rate, often experienced in such intraplate setting. The study exemplifies the fluvial response, of transport deficit dryland environments, to external perturbations and their application in reconstructing palaeo extreme events.

Evidence for slow late-glacial ice retreat in the upper Rangitata Valley, South Island, New Zealand

A suite of cosmogenic radionuclide ages taken from boulders on lateral and latero-terminal moraines in the Rangitata Valley, eastern South Island, New Zealand demonstrates that relatively thick ice occupied valley reaches inland of the Rangitata Gorge until c. 21 ka. Thereafter ice began to thin, and by c. 17 ka it had retreated 33 km up-valley of the Rangitata Gorge to the Butler-Brabazon Downs, a structurally created basin in the upper Rangitata Valley. Despite its magnitude, this retreat represents a minor ice volume reduction from 21 ka to 17 ka, and numerous lateral moraines preserved suggest a relatively gradual retreat over that 4 ka period. In contrast to records from adjacent valleys, there is no evidence for an ice-collapse at c. 18 ka. We argue that the Rangitata record constitutes a more direct record of glacial response to deglacial climate than other records where glacial dynamics were influenced by proglacial lake development, such as the Rakaia Valley to the North and the major valleys in the Mackenzie Basin to the south-west. Our data supports the concept of a gradual warming during the early deglaciation in the South Island New Zealand.