Ice-rafted Debris Research Articles

Three-phase structure of the East Asia summer monsoon during Heinrich Stadial 4 recorded in Xianyun Cave, southeastern China

The Heinrich Stadial (HS) 4 event was the largest of the six HS events and occurred from 40.0 to 38.2 kyr BP (thousand years before present, where present = 1950 CE) recorded in Greenland ice cores. However, its structure, forcings, and related global hydroclimate variation remain poorly understood. Here, an 8-yr-resolved stalagmite δ18O record from Xianyun Cave, southeastern China, is used to reconstruct the multidecadal-to-millennial-scale changes of the East Asian summer monsoon (EASM) from 41.33 ± 0.09 to 37.04 ± 0.05 kyr BP. The Xianyun record shows a corresponding weak monsoon interval from 40.04 ± 0.07 to 37.76 ± 0.05 kyr BP during HS 4 and is the first direct evidence in the Asian monsoon realm to clearly reveal a three-phase EASM structure. The first phase began with a weakening monsoon from 40.04 ± 0.07 to 39.54 ± 0.06 kyr BP, associated with a southward shift of the Intertropical Convergence Zone (ITCZ). The second phase was a long weak monsoon, spanning 1.06 kyr from 39.54 ± 0.06 to 38.48 ± 0.05 kyr BP and corresponding to an ice-rafted debris event in the North Atlantic Ocean and an intense pluvial phase in Northeast (NE) Brazil, suggesting that the ITCZ lingered at its southernmost position during this interval. Following the end of this phase, EASM was gradually enhanced for 720 yrs through 37.76 ± 0.05 kyr BP. The feature expressed in the Xianyun record agrees with the variations in low-latitude climate signals recorded in the Northern Greenland Eemian Ice Drilling (NEEM) 17O-excess and anti-phase matches with dry-wet transition recorded in NE Brazilian stalagmites. These relations suggest multi-phased responses of tropical atmospheric/oceanic circulations to forcing from northern hemisphere high latitudes. Our findings offer essential evidence for understanding low- and high-latitude climate teleconnections and the relationship between the monsoon and the tropical hydrologic cycle.

Controls on Terrigenous Detritus Deposition and Oceanography Changes in the Central Okhotsk Sea Over the Past 1550 ka

The Okhotsk Sea, which connects the high latitude Asian continent and the North Pacific, plays an important role in modern and past climate changes due to seasonal sea ice coverage and as a precursor of the North Pacific Intermediate Water. The long-term glacial-interglacial changes of sea ice coverage and its impacts on terrigenous transport and surface primary productivity in the Okhotsk Sea remain, however, not well constrained. Base on the paleomagnetic, rock magnetic, micropaleontological (diatom), and geochemical studies of the marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth: 1,123 m) taken in the central Okhotsk Sea, we reconstruct the terrigenous sediment transport and paleoceanographic variations during the past 1550 thousand years (kyr). Seventeen geomagnetic excursions are identified from the paleomagnetic directional record. Close to the bottom of the core, an excursion was observed, which is proposed to be the Gilsa event ∼1550 thousand years ago (ka). During glacial intervals, our records reveal a wide extension of sea ice coverage and low marine productivity. We observed ice-rafted debris from mountain icebergs composed of coarse and high magnetic terrigenous detritus which were derived from the Kamchatka Peninsula to the central Okhotsk basin. Still during glacial intervals, the initiation (i.e., at ∼900 ka) of the Mid-Pleistocene Transition marks the changes to even lower marine productivity, suggesting that sea-ice coverage became larger during the last 900 ka. During interglacial intervals, the central Okhotsk Sea was either devoid of sea-ice or the ice was at best seasonal; resulting in high marine productivity. The weaker formation of Okhotsk Sea Intermediate Water, lower ventilation, and microbial degradation of organic matter depleted the oxygen concentration in the bottom water and created a reduced environment condition in the sea basin. The freshwater supplied by snow or glacier melting from Siberia and Kamchatka delivered fine grain sediments to the Okhotsk Sea. During the stronger interglacial intervals after the Mid-Brunhes Transition (i.e., Marine Isotope Stages 1, 5e, 9, and 11), strong freshwater discharges from Amur River drainage area are in association with intensified East Asian Summer Monsoon. This process may have enhanced the input of fine-grained terrigenous sediments to the central Okhotsk Sea.

Late Quaternary Abrupt Climate Change in the Tropics and Sub‐Tropics: The Continental Signal of Tropical Hydroclimatic Events (THEs)

AbstractTropical hydroclimatic events, characterized by extreme regional rainfall anomalies, were a recurrent feature of marine isotope stages 2–4 and involved some of the most abrupt and dramatic climatic changes in the late Quaternary. These anomalies were pervasive throughout the tropics and resulted from the southward displacement of the Hadley circulation and the Intertropical Convergence Zone (ITCZ) and its associated convective rainfall, modulated by regional factors. Lake sediments, stalagmites, and offshore marine sediments that integrate inland continental conditions provide a comprehensive record of these changes over the past ∼70,000 yr. Vast areas experienced severe drought while other areas recorded greatly increased rainfall. Within the uncertainties of dating, these tropical rainfall anomalies occurred very close in time (±102–103 yr) to the deposition of North Atlantic ice‐rafted debris (IRD) that defines Heinrich events (HEs). The IRD record is a good proxy for the amount and distribution of additional freshwater forcing which was necessary to bring about a drastic reduction in the Atlantic Meridional Overturning Circulation (AMOC) strength during each HE. As a consequence of this reduction in AMOC and an abrupt expansion in the area of sea‐ice, cooling of the North Atlantic and adjacent continents took place, with a rapid atmospheric response involving the southward displacement of the ITCZ and associated rainfall belts. The climatic consequences of this large‐scale change in the Hadley circulation, modulated by regional factors, is clearly recorded throughout the tropics as a series of abrupt and extreme hydroclimatic events. Some of the physical mechanisms that may have played a role in those changes are discussed.

Pleistocene depositional environments and links to cryosphere-ocean interactions on the eastern Ross Sea continental slope, Antarctica (IODP Hole U1525A)

The repeated proximity of West Antarctic Ice Sheet (WAIS) ice to the eastern Ross Sea continental shelf break during past ice age cycles has been inferred to directly influence sedimentary processes occurring on the continental slope, such as turbidity current and debris flow activity; thus, the records of these processes can be used to study the past history of the WAIS. Ross Sea slope sediments may additionally provide an archive on the history and interplay of density-driven or geostrophic oceanic bottom currents with ice-sheet-driven depositional mechanisms. We investigate the upper 121 m of Hole U1525A, collected during International Ocean Discovery Program (IODP) Expedition 374 in 2018. Hole U1525A is located on the southwestern external levee of the Hillary Canyon (Ross Sea, Antarctica) and the depositional lobe of the nearby trough-mouth fan. Using core descriptions, grain size analysis, and physical properties datasets, we develop a lithofacies scheme that allows construction of a detailed depositional model and environmental history of past ice sheet-ocean interactions at the eastern Ross Sea continental shelf break/slope since ~2.4 Ma. The earliest Pleistocene interval (~2.4- ~ 1.4 Ma) represents a hemipelagic environment dominated by ice-rafting and reworking/deposition by relatively persistent bottom current activity. Finely interlaminated silty muds with ice-rafted debris (IRD) layers are interpreted as contourites. Between ~1.4 and ~0.8 Ma, geostrophic bottom current activity was weaker and turbiditic processes more common, likely related to the increased proximity of grounded ice at the shelf edge. Silty, normally-graded laminations with sharp bases may be the result of flow-stripped turbidity currents overbanking the canyon levee during periods when ice was grounded at or proximal to the shelf edge. A sandy, IRD- and foraminifera-bearing interval dated to ~1.18 Ma potentially reflects warmer oceanographic conditions and a period of stronger Antarctic Slope Current flow. This may have enhanced upwelling of warm Circumpolar Deep Water onto the shelf, leading to large-scale glacial retreat at that time. The thickest interval of turbidite interlamination was deposited after ~1 Ma, following the onset of the Mid-Pleistocene Transition, interpreted as a time when most ice sheets grew and glacial periods were longer and more extreme. Sedimentation after ~0.8 Ma was dominated by glacigenic debris flow deposition, as the trough mouth fan that dominates the eastern Ross Sea continental slope prograded and expanded over the site. These findings will help to improve estimations of WAIS ice extent in future Ross Sea shelf-based modelling studies, and provide a basis for more detailed analysis of the inception and growth of the WAIS under distinct oceanographic conditions.

Semi‐Automatic Ice‐Rafted Debris Quantification With Computed Tomography

AbstractSedimentary ice‐rafted debris (IRD) provides critical information about the climate sensitivity and dynamics of ice sheets. In recent decades, high‐resolution investigations have revelated ice rafting events in response to rapid warming: such reconstructions help us constrain the near‐future stability of our planet's fast‐changing cryosphere. However, similar efforts require laborious and destructive analytical procedures to separate and count IRD. Computed tomography (CT) holds great promise to overcome these impediments to progress by enabling the micrometer‐scale (max. ∼21 μm) visualization of individual IRD grains. This study demonstrates the potential of this emerging approach by (a) validating CT counts in synthetic sediment archives (phantoms) spiked with a known number of grains, (b) replicating published IRD stratigraphies, and (c) improving sampling resolution. Our results show that semi‐automated CT counting of grains in the often analyzed 150–500 μm size fraction reproduces grain numbers and tracks manually counted trends. We also find that differences between manual and CT‐counted data are explained by image processing artifacts, offsets in sampling resolution, and bioturbation. By acquiring these promising results using basic image processing tools, we argue that our work advances and broadens the applicability of ultra‐high resolution IRD counting with CT to deepen our understanding of ice sheet‐climate interactions on human‐relevant timescales.

Late Quaternary high-resolution seismic stratigraphy and core-based paleoenvironmental reconstructions in Ona Basin, southwestern Scotia Sea (Antarctica)

The variability of sedimentation patterns and processes driven by late Quaternary glacial-interglacial paleoclimatic and paleoceanographic changes are investigated in Ona Basin, southwestern Scotia Sea. The interest of this area lies in the fact that the nearby Antarctic Peninsula has recorded extreme climatic variability, and the Drake Passage-Scotia Sea oceanic domain is influenced by two major Southern Ocean water masses, the eastward-flowing Antarctic Circumpolar Current (ACC) and the westward-flowing Weddell Sea Deep Water (WSDW). These goals are achieved through the examination of a grid of very high-resolution sub-bottom profiles and two gravity cores collected in Ona Basin. Multi-proxy data derived from the gravity cores include 14C-derived ages, descriptions of sedimentary units and diatom assemblages, and continuous logging of physical properties and micro-XRF core scanning.The sub-surface seismic stratigraphy is composed of four seismic units (U4 to U1) with a dominant sub-parallel configuration, with local occurrence of wavy facies and intercalations of transparent seismic facies. Additionally, four sedimentary units were recognized through sediment core analysis from bottom to top: Unit IV is composed of slightly bioturbated diatom-rich mud and silty mud with sparse ice-rafted debris (IRDs); Unit III is composed of gravelly silty to sandy mud with large amounts of IRDs; Unit II mostly contains bioturbated diatomaceous mud; and Unit I is composed of diatom-rich silty to sandy mud. The highest diatom abundances are found in Unit II, whereas highly variable abundances are found in Unit IV. The most common diatoms are Fragilariopsis kerguelensis and Chaetoceros subg. Hyalochaete. Overall, these characteristics document a change in the depositional style from terrigenous during the Last Glacial Maximum (LGM) to hemipelagic sedimentation during the deglaciation.The high-resolution seismic stratigraphy analysis reveals significant fluctuations in the regional bottom-current patterns during the late Quaternary (i.e., after 0.4 Ma) glacial-interglacial cycles. An overall strengthening of the westward-flowing WSDW is postulated in relation to latitudinal displacements of the interphase between the deeper ACC and the WSDW, together with enhanced interactions between along- and downslope processes. In addition, sedimentological, geochemical, and micropaleontological analyses revealed two distinctive phases during the late Pleistocene, in terms of paleoenvironments and paleoceanographic conditions. During the LGM, extensive sea-ice coverage limited biogenic productivity in the ocean. Increased terrigenous input was largely supplied by the westward-flowing WSDW, under a reduced ACC influence due to the northward location of fronts. During deglaciation, the sediment record indicates reduced sea-ice cover and increased open-ocean conditions and surface water productivity, as well as a long-term intensification of the WSDW flow. We postulate bottom-current strengthening was driven by an increased Weddell Sea water export and the southward migration of fronts as a consequence of major retreat of sea ice, enhancing the ACC influence in the southern Ona Basin, and thus, affecting the sloping interphase between the deeper ACC and the WSDW.

Change in Sediment Provenance on the Inner Slope of the Chukchi Rise and Their Paleoenvironmental Implications

The Arctic Ocean is one of the world’s most remarkable regions with respect to global climate change. The core ARA09C-St03 was analyzed for mineral composition and Nd isotope to determine the sediment provenance and reconstruct the paleoenvironment in the inner slope of the Chukchi Rise. Core ARA09C-St03 represents overall cycles of brown and gray color with three distinct dark brown layers and two pinkish-white layers and is divided into eight sedimentary units based on the lithological feature. The core has a continuous record of the late marine isotope stage (MIS) 5 to the Holocene and in particular provides a particularly high-resolution record from the Last Glacial Maximum (LGM). Sediment is derived mainly from the adjacent East Siberian Sea and the North American region, and changes in sediment provenance are controlled by climate-dependent particle size. During the glacial/stadial periods, sediments in Units 3, 5, and 8 were supplied by the East Siberian Sea via meltwater-derived suspension. The major ice-rafted debris (IRD) events in Units 2, 4, and 7, characterized by abundant dolomite and K/C ratio, were sourced from North America. The North America-derived materials reflect the initiation and disintegration of the Laurentide Ice Sheet and icebergs transported them across the open Arctic Ocean. The differences in provenance within these periods may be related to the scale of the Laurentide Ice Sheet. Interglacial sediments, including those from Units 1 and 6, are of mixed origin from Eurasia and the Canadian Archipelago and may have been transported by oceanic current and seasonal sea ice. These periods are likely associated with the negative Arctic Oscillation (AO) intensifying the Beaufort Gyre.

Orbital and Millennial Variations in Sea Ice in the Southwestern Okhotsk Sea Since the Last Interglacial Period and Their Implications

Sea ice in the Okhotsk Sea plays a significant role in global climate change. However, the history and mechanism of changes in sea ice spanning the last glacial cycle remain controversial. In this study, an 8.8 m core (LV55-40-1) was recovered from the southwestern Okhotsk Sea that contains a continuous sea ice record over the past ∼110 kyr. The sand fraction and dropstones were used as ice-rafted debris proxies to reconstruct the history of sea ice variations over the last ∼110 kyr and to determine the underlying causes on orbital and millennial timescales. Sea ice expansions occurred during MIS 5b, MIS 4, mid-MIS 3, and early MIS 1, which were controlled mainly by decreased autumn insolation on an orbital timescale. Superimposed on the orbital-scale changes, millennial-scale variations in sea ice were also observed, with 19 expansion events that coincided with cold Dansgaard-Oeschger stadials. Millennial scale sea ice variations were most likely controlled by both the Arctic oscillation and the East Asian summer monsoon. During periods of negative Arctic oscillation patterns, decreased air temperatures over the Okhotsk Sea caused more active sea ice formation. Such conditions could have been reinforced, by a reduced influence of warm advection at the surface of the Okhotsk Sea caused by decreased discharge from the Amur River that resulted from a weakened East Asian summer monsoon during cold stadials.

Biogenic carbonate fluxes and preservation in the northwestern Labrador Sea since the Last Glacial Maximum

The important changes that took place in the glacial cycle at the termination, from the Last Glacial Maximum to the present interglacial, deserve an examination of ocean sedimentary records that document past productivity, carbon fluxes, and carbonate preservation. In this study, we analyzed coccoliths, alkenones, and foraminifers in core HU2008–029-004 PC (61.46°N and 58.04°W, water depth = 2,674 m) from the northwestern Labrador Sea to document linkages between hydrographic conditions, biogenic carbonate fluxes to the seafloor, and their preservation/dissolution during the last 25,000 years. Large changes in coccolith and foraminifer concentrations are recorded, with sediments from the last glacial interval containing significantly less carbonate microfossils (9.5 ± 3.9 × 105 coccoliths g−1 and 2,860 ± 580 planktonic foraminifers g−1) than sediments from the deglacial and postglacial intervals (up to 3.1 × 108 coccoliths g−1 and 2.9 × 104 foraminifers g−1). Three foraminifer-based calcite dissolution indices were used to evaluate biogenic carbonate preservation: the planktonic foraminifer fragmentation index, the ratio of benthic-to-planktonic foraminifers (B/P), and the ratio of organic linings to benthic foraminifers (OL/B). Fragmentation remained low throughout the postglacial (mean of 4%) but reached up to 8% in the deglacial and peaked at 16% in samples from the Bølling-Allerød of the late glacial interval. Samples from the Bølling-Allerød and the deglacial interval also display a slightly elevated B/P index (>0.15), which suggests that some dissolution may have occurred. In contrast, with the exception of the Bølling-Allerød and the deglacial interval, near zero OL/B values characterize most of the sequence, suggesting good biogenic carbonate preservation, which implies that the low biogenic carbonate and coccolith content in sediments of the glacial stage mirror low productivity of calcifying organisms. The elevated fragmentation of foraminifers during the Bølling-Allerød and the deglacial interval, a time of elevated productivity and low percentages of ice-rafted debris, may indicate the development of calcite undersaturated porewaters and consequent dissolution resulting from oxic remineralization of sedimentary organic matter. We also identify a significant decoupling of coccolith and alkenone concentrations throughout the core. Colder-than-expected UK37-SST estimates from the alkenones of the glacial interval rule out possible allochthonous inputs from lower-latitude locations. Instead, our records imply that at least during the glacial interval, alkenones were produced by non-calcifying haptophytes that may not follow the canonical UK37-based temperature calibrations.

Tephrochronological constraints on the timing and nature of sea-level change prior to and during glacial termination V

Glacial-interglacial variations in ice volume and sea level are essential components of the Pleistocene global climate evolution. Deciphering the timing of change of these key climate parameters with respect to the insolation forcing is central to understanding the processes controlling glacial terminations. Here we exploit the sensitivity of the Paleo Tiber River (central Italy) to sea-level forced changes in the base level and the frequent occurrence of datable tephra layers in its sedimentary successions to reconstruct the timing of the relative sea-level (RSL) change between 450 and 403 ka, i.e., across the glacial termination (T-V) that marks the transition between Marine Isotope Stage (MIS) 12 and MIS 11. The analysis hinges on new stratigraphic data, tephra geochemical fingerprinting, and 40Ar/39Ar dating from a fluvial section that represents the inland counterpart of the near mouth, coastal aggradational successions of the San Paolo Formation (SPF). Tephra correlation indicates that the morpho-stratigraphic record of the inland section is as sensitive to the sea-level change as its coastal counterparts, which makes it ideal to complement previous RSL reconstructions from the Tiber River catchment basin, thereby providing a more detailed picture of the sea-level history across T-V. Combined sedimentological and morphological proxies of the composed inland-coastal SPF record document the occurrence of two phases of relatively rapid sea-level rise, here interpreted as meltwater pulse (MWP) events. The earlier MWP occurred between ∼450 and ∼445 ka and matches a relatively minor episode of the sea-level rise documented in an existing RSL record, while the younger MWP at ∼430 ka corresponds to the high amplitude sea-level rise that marks T-V. We find that both MWPs coincide with episodes of ice-rafted debris deposition in the North Atlantic (Heinrich-like events) and with attendant Southern Hemisphere warming, plausibly associated with the bipolar seesaw.

Spatial Distribution and Provenance of Detrital Minerals of Surface Sediment in the Okhotsk Sea

The Okhotsk Sea is a distinctive marginal sea in the northwestern Pacific Ocean, which is characterized by the prevalence of seasonal sea ice in winter. Sediment is sourced from the around region through sea ice transportation, rivers input, and volcanic eruptions. Surface sediments of the Okhotsk Sea shelf vary greatly in grain size, and the sand content is generally high, which is conducive to source-to-sink studies using ice-rafted detritus (IRD),detrital minerals and single mineral geochemistry methods. In this paper, the 63–125 μm grain size fraction was selected for the detrital minerals analysis of surface sediments (top 0–10 cm) from 58 sediment stations and 15 stations samples has been chosen for garnet chemistry. These stations are mainly located in the south central Okhotsk Sea. The distribution and composition of the heavy minerals are influenced by material derived from the Amur River, the north shelf (Okhotsk-Chukotka volcanic belt), Sakhalin Island, the Kamchatka Peninsula, and the Kuril Islands. The detrital mineral results show that hornblende, epidote and garnet are terrigenous material indicators. High contents of fresh hypersthene can be used as an indicator of volcanic eruption materials. And high content of abraded hypersthene can be used as an indicator of Okhotsk-Chukotka volcanic materials. In the northern Okhotsk Sea, the southward moving sea ice produced in Sakhalin Bay collides with the sea ice produced off the east coast of Sakhalin Island, which causes the sea ice to accumulate to the southeast. This results in the deposited ice-rafted debris having a southeastward facing fan shape, and the geochemical analysis of the garnet supports this conclusion. From west to east, the amount of material from Sakhalin Island gradually decreases, while the amount of material from the Amur River and Chukchi-Kamchatka increases.

A ~240 ka record of Ice Sheet and Ocean interactions on the Snorri Drift, SW of Iceland

Core MD99-2323 was extracted from the Snorri Drift at a water depth of 1062 m, just south of the Denmark Strait, and ~120 km from the Last Glacial Maximum (LGM) margins of the Iceland and East Greenland Ice Sheets. The core chronology (~7.5 to 240 cal ka) is derived from radiocarbon dates, marker tephra, paleomagnetic excursion, and correlation with North Atlantic δ18O records on Neogloboquadrina pachyderma (δ18ONp). Sedimentation averaged ~7.5 cm/kyr. Records of proxy flow speed, ice rafted debris (IRD) and oxygen isotopes show that many IRD abundance peaks represent winnowing of the fine fraction by faster flows rather than pulses of increased IRD flux. The overall pattern of flow speed does not resemble the classic fast interglacial/slow glacial pattern seen in records of Nordic Sea overflow, rather the current record is suggested to be partly controlled by the production of brine-driven gravity flows from adjacent ice fronts, especially during cold periods. On a smaller scale the usual glacial/slow – interglacial/fast pattern appears to be the case during ~5 kyr oscillations during Marine Isotope Stage (MIS) 6 where periodic low flow speed is matched by high values of planktonic oxygen isotope ratios. Eight peaks in quartz wt% reflect increased contributions from glacial erosion of Precambrian and Caledonian bedrock from E and NE Greenland; peaks in dolomite may reflect glacial-marine transport from the Laurentide Ice Sheet. Cross wavelet analysis of the δ18ONp versus sortable silt and quartz records indicate significant precession and obliquity periodicities, but with little temporal correlations due to leads and lags in responses.

Enhanced iceberg discharge in the western North Atlantic during all Heinrich events of the last glaciation

A series of catastrophic iceberg and meltwater discharges to the North Atlantic, termed Heinrich events, punctuated the last ice age. During Heinrich events, coarse terrigenous debris released from the drifting icebergs was preserved in deep-sea sediments, serving as an indicator of iceberg passage. Quantifying the vertical flux of ice-rafted debris (IRD) in open-ocean settings can resolve questions regarding the timing and spatial variation in ice sheet calving intensity. In this study, 230Thxs-based IRD flux throughout the last glacial period was measured in a deep-sea sediment core from the western North Atlantic, and complemented by data spanning 0-32 ka from a sediment core in the Labrador Sea. The cores were recovered from sites downstream from Hudson Strait, a likely conduit for icebergs calving from the Laurentide ice sheet (LIS). We compare our results with equivalent existing data from the eastern North Atlantic and show that the two cores in our study have higher IRD fluxes during all Heinrich events, notably including events H3 (∼31 ka) and H6 (∼60 ka). This study demonstrates that the LIS played a role in all Heinrich events, and raises the likelihood that a single mechanism can account for the genesis of these events.

Last Glacial Maximum to Holocene paleoceanography of the northwestern Ross Sea inferred from sediment core geochemistry and micropaleontology at Hallett Ridge

Abstract. During the Late Pleistocene–Holocene, the Ross Sea Ice Shelf exhibited strong spatial variability in relation to the atmospheric and oceanographic climatic variations. Despite being thoroughly investigated, the timing of the ice sheet retreat from the outer continental shelf since the Last Glacial Maximum (LGM) still remains controversial, mainly due to a lack of sediment cores with a robust chronostratigraphy. For this reason, the recent recovery of sediments containing a continuous occurrence of calcareous foraminifera provides the important opportunity to create a reliable age model and document the early deglacial phase in particular. Here we present a multiproxy study from a sediment core collected at the Hallett Ridge (1800 m of depth), where significant occurrences of calcareous planktonic and benthic foraminifera allow us to document the first evidence of the deglaciation after the LGM at about 20.2 ka. Our results suggest that the co-occurrence of large Neogloboquadrina pachyderma tests and abundant juvenile forms reflects the beginning of open-water conditions and coverage of seasonal sea ice. Our multiproxy approach based on diatoms, silicoflagellates, carbon and oxygen stable isotopes on N. pachyderma, sediment texture, and geochemistry indicates that abrupt warming occurred at approximately 17.8 ka, followed by a period of increasing biological productivity. During the Holocene, the exclusive dominance of agglutinated benthic foraminifera suggests that dissolution was the main controlling factor on calcareous test accumulation and preservation. Diatoms and silicoflagellates show that ocean conditions were variable during the middle Holocene and the beginning of the Neoglacial period at around 4 ka. In the Neoglacial, an increase in sand content testifies to a strengthening of bottom-water currents, supported by an increase in the abundance of the tycopelagic fossil diatom Paralia sulcata transported from the coastal regions, while an increase in ice-rafted debris suggests more glacial transport by icebergs.

The main periods and environmental controls of coastal dune development along the west coast of the Korean Peninsula during the mid to late Holocene

This study traces coastal sand dune development processes in East Asia during the Holocene to understand climate and environmental changes. We used optically stimulated luminescence (OSL) dating to examine periods of activation and stabilization of dunes along the western coast of the Korean Peninsula. The results from the Taean Chollipo, Seocheon Dasari, and Gochang Sabanri dunes were then compared with those from previous studies to establish a broad understanding of coastal dune development in this region. Sedimentation rates were calculated for the three dunes, and their development periods were classified through statistical analysis that included data from previous studies. We defined five main periods of late Holocene dune development (<0.1 ka, 0.2–0.5 ka, 1.2–2.0 ka, 2.5–3.2 ka, and 5.5–6.5 ka). These correlated to periods of sea level decrease and global cooling determined by ice rafted debris records. We concluded that the development of coastal dunes in western Korea was influenced by sea level decreases, and global cooling events were enhanced by the East Asian winter monsoon. In addition, human activity increases also contributed to the development of the dunes. This study showed how coastal sand dunes can be evaluated as important indicators of global climate change. These results improved our understanding of how paleoclimate changes affected environmental conditions in East Asia and how coastal dune development periods can be used to reconstruct paleo-environmental conditions.

Exploring the extent to which fluctuations in ice‐rafted debris reflect mass changes in the source ice sheet: a model–observation comparison using the last British–Irish Ice Sheet

ABSTRACTThe British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore ice‐rafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity iceberg–climate model, with varying BIIS catchment‐level iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000‐year time steps during the height of the last BIIS glaciation (31–6 ka bp). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances.

Stratigraphy and U–Pb geochronology of the basal units of the vazante group: A lateral correlation with the glaciogenic Jequitaí formation (Minas Gerais, Brazil)

The basal units of the Vazante Group, the Santo Antônio do Bonito and Rocinha formations, present different stratigraphic records and younger ages than the rest of the group. These units are characterized by diamictites, sandstones, pelites and rare carbonates originated from subaqueous gravitational sedimentation. U–Pb geochronological data in detrital zircon indicate a Tonian maximum depositional age for both formations. The youngest zircon grains yielded ages ranging from 903 to 935 Ma. The main source for zircon is Paleoproterozoic (mainly Rhyacian and Orosirian) to Mesoproterozoic (Calymmian and Ectasian) areas being probably the São Francisco paleocontinent the main area. Due to the complete absence of ice-rafted debris within fine-grained sediments in the Santo Antônio do Bonito Formation, we interpret a gravitational reworking as the main process of diamictites deposition, probably during a period of ice retreat and sea level rise. The U–Pb data corroborate a lateral correlation between the Santo Antônio do Bonito and Rocinha formations with the glaciogenic Jequitaí Formation (Bambuí Group). From east to west, glacial sediments of the Jequitaí Formation were reworked probably by a slope-apron system during a late-glacial to post-glacial stage on the western margin of the São Francisco paleocontinent. Therefore, the Vazante Group can be divided into two successions: the upper Lagamar-Serra do Garrote formations and the lower Santo Antônio do Bonito-Rocinha formations. The Lagamar-Serra do Garrote sedimentation comprises a late Mesoproterozoic mixed carbonate-siliciclastic deposition in passive margins. The second succession is a gravitational slope-apron resedimentation and probably correlates with the Neoproterozoic glacial rocks of the Jequitaí Formation from the Bambuí Group. Finally, the collisional phase of the Brasília Belt was responsible for stratigraphic inversion, placing the older Lagamar-Serra do Garrote formations over the younger Santo Antônio do Bonito-Rocinha formations.

Quantification of temperature and precipitation changes in northern China during the “5000-year” Chinese History

High-quality paleoclimate reconstructions can provide crucial climate context to test the hypothesis of climatic impact on historical culture changes. Here we report high-resolution and quantitative temperature and precipitation records covering the entire “5000-year” Chinese history in northern China. Our temperature record shows a slight decrease before 1800 cal yr BP and a ∼4 °C rapid cooling afterwards, superimposed with four major ∼2–3 °C centennial-scale cold events, potentially corresponding to the widespread North Atlantic ice-rafted debris Bond events. While precipitation record shows high value before 3000 cal yr BP, and a gradual decrease of ∼250 mm with two distinct ∼100 mm centennial-scale dry intervals after 1100 cal yr BP. Our data not only provide a more complete climate background for Chinese dynasties, but show the coincidence in the timing between abrupt cold and/or dry events and large-scale social unrests and southern migration of nomads. This finding reveals climate fluctuations, in particular variations in temperature, played an important role in affecting Chinese historical cultural changes.

Millennial‐Scale Changes in Bottom Water Temperature and Water Mass Exchange Through the Fram Strait 79°N, 63‐13 ka

AbstractThe Svalbard margin, in the eastern Fram Strait with its high sediment accumulation, form a key area for the reconstruction of water mass and heat exchange between the North Atlantic and Arctic Ocean in relation to abrupt climate changes as seen in glacial Greenland Interstadial and Greenland Stadial (GI‐GS) events. Here, we present a bottom water temperature (BWT) record from the northern Nordic Seas (79°N) at 1,273 m water depth based on benthic foraminiferal Mg/Ca. The BWT reconstructions, combined with benthic foraminiferal stable isotopes, benthic foraminiferal fauna compositions and ice‐rafted debris (IRD), reveal at least two distinctive scenarios for the GI‐GS events during the last glacial period (13–63 ka). During GIs, conditions were similar to modern with high productivity, low BWT and deep convection. During GS6, GS8, and GS15 and during Heinrich Stadials (HSs), BWT increased up to 5°C ± 1°C generally concomitant with low planktic and benthic δ18O. Our results suggest, that during some GSs and HSs, deep water generation was reduced, allowing the subsurface Atlantic water (AW) to thicken and deepen down to at least the core site depth. A strong halocline during HSs and GSs prevented heat release from the subsurface AW, which we can now trace from 45°N in the North Atlantic to the Arctic Ocean &gt;79°N. Surfacing of the salty Atlantic subsurface water preconditioned the Nordic seas for convection. Release of the subsurface heat from this vast reservoir must have contributed to the large and abrupt atmospheric warmings at the start of GIs.

Rapid Tephra Identification in Geological Archives With Computed Tomography: Experimental Results and Natural Applications

Volcanic ash (tephra) horizons represent powerful chronological and stratigraphic markers: rapid and widespread deposition allows for correlation of geological records in time and space. Recent analytical advances enable identification of invisible ash (cryptotephra) up to thousands of kilometers from its volcanic source. This momentum has greatly expanded the reach and potential of tephrochronology: some deposits can now be traced across continents and oceans. However, the laborious laboratory procedures required to identify tephra horizons in geological archives hold back the pace of progress. By allowing the rapid visualization of ash at micrometer (µm) scales, computed tomography (CT) holds great promise to overcome these restrictions. In this study, we further demonstrate the potential of this tool for the tephra community with experimental results and applications on conventionally analyzed archives. A custom-made scanner helps us strike a balance between the convenience of whole-core medical scanners and the µm-resolution of micro-CT systems. Using basic image processing tools that can be readily mastered by tephrochronologists, we identified invisible horizons down to ∼500 shards in synthetic cores. In addition, procedures for the removal of image artifacts can be used to visualize other paleoenvironmental indicators such as bioturbation burrows, ice rafted debris or mineral dust. When applied on segments of manually counted natural archives, our approach captures cryptic glass shard maxima down to ∼300 shards/cm3. We also highlight the value of CT to help optimize sampling strategies by identifying micrometer-scale ash horizons that were not detected in shard count profiles. In conclusion, this work helps broaden the applicability of CT as a promising frontier in tephrochronology that can advance the field by optimizing the efficiency and accuracy of isochron detection.