Heinrich Events Research Articles

Hydroclimate variability in the Caribbean during North Atlantic Heinrich cooling events (H8 and H9)

We present a speleothem record from western Cuba, spanning the period 98.7–84.9 ka BP. Our record shows two distinctive periods of high δ18O corresponding to dry and/or cold periods during 85–87.6 and 90.2–93.1 ka BP, synchronous with Heinrich events 8 and 9 (H8 and H9). Hence, we provide the first proxy evidence of the local Caribbean climate response to H8 and H9. Interestingly, H8 is more pronounced compared to H9, which may be a local response to lower temperatures in the North Atlantic resulting in a weak AMOC and reduced deep water formation, therefore a stronger south shift of the ITCZ. Our data complement existing speleothem records from western Cuba which, collectively, provide a nearly continuous paleoclimate time-series spanning the last 100 ka BP, indicating a consistent response to millennial-scale events as dry and/or cooler conditions. The comparison with regional paleoclimate records reveals an anti-phased relationship with South America, caused by the southern movements of the ITCZ during millennial-scale events which lead to dry conditions in the Caribbean and a stronger South American Monsoon System.

Asian monsoon intensity coupled to Antarctic climate during Dansgaard–Oeschger 8 and Heinrich 4 glacial intervals

The transition from glacial to interglacial periods has been hypothesized to be linked to millennial-scale changes in oceanic/atmospheric circulation, but the relationships between these phenomena remain poorly constrained. Here we present a speleothem oxygen isotope record from Yongxing Cave, China, spanning 40.9 to 33.1 ka and compare this to existing Antarctic proxy records. We find that decadal-to-centennial rapid shifts in the Asian summer monsoon, Antarctic temperature, atmospheric methane and carbon dioxide are all coupled together during Dansgaard–Oeschger cycles, which may suggest an important role of the Intertropical Convergence Zone and Southern Ocean in driving the global greenhouse gas changes. Analogous to millennial-scale variations in trend, amplitude and internal sub-centennial-scale structures during Dansgaard–Oeschger 8 and Heinrich Stadial 4, the Younger Dryas and Heinrich Stadial 1 during the last ice termination provided critical positive feedbacks to changes in terrestrial vegetation and northern ice volume, and may have contributed to glacial to interglacial transition.

Variations in Asian summer monsoon and hydroclimate during Heinrich stadials 4 revealed by stalagmite stable isotopes and trace elements

The iceberg melting-induced Heinrich events (HEs), which were first reported in 1988, caused a series of centennial-millennial global climate changes during the last glacial period. The patterns and details of low-latitude monsoons, oceans, and atmospheric circulation responses to HEs still need to be understood. Here, we report 11a resolved δ13C, δ18O and 36a resolved Mg/Ca, Sr/Ca and Ba/Ca multi-proxy stalagmite records during 42.29–37.14 ka BP from Yangkou cave, southwestern China. The Greenland ice cores and the δ18O of stalagmites from northern China recorded a rapid onset/termination of Heinrich stadials (HS) 4 (∼0.2 ka). However, the transition interval for the weakening/strengthening of the Asian summer monsoon (ASM) during HS4, as recorded by the southern Chinese stalagmite δ18O, is about 0.5 ka. Low latitude monsoons and hydrological variations are influenced by tropical oceans and interhemispheric thermal contrasts, which prolong the onset/termination transition process of HS4. Our findings highlight the importance of atmosphere-ocean interactions at low latitudes and in the Southern Hemisphere for ASM dynamics, which triggered the termination of millennial-scales abrupt climate events. During HS4, variations of the δ13C and trace elements ratios are inconsistent with δ18O, indicating that while the ASM is weakened, regional hydroclimate and local ecology are not significantly degraded, resulting in a relatively moderate hydrological environment. Prolonged southerly subtropical westerlies, super El Niño Southern Oscillation, western China autumn rains, and lower regional evaporation, may increase regional precipitation in different seasons and regulate hydrological conditions during HS4. As a result, while the ASM has weakened, regional hydrological conditions in the Yangtze River basin in southwest China maintain wet environmental conditions. The gradual northward shift of the Intertropical Convergence Zone and subtropical westerlies during the latter phase of HS4 will cause a limited northward shift of the rainfall bands. Although regional hydrological conditions deteriorate slightly in comparison to the early phase, it remains humid overall. We present new evidence that local hydrological conditions and ASM intensity were not fully coupled during the abrupt climate events on millennial scales in the last glacial period.

Antiphase response of the Indonesian–Australian monsoon to millennial-scale events of the last glacial period

Antiphase behaviour of monsoon systems in alternate hemispheres is well established at yearly and orbital scales in response to alternating sensible heating of continental landmasses. At intermediate timescales without a sensible heating mechanism both in-phase and antiphase behaviours of northern and southern hemisphere monsoon systems are recorded at different places and timescales. At present, there is no continuous, high resolution, precisely dated record of millennial-scale variability of the Indonesian–Australian monsoon during the last glacial period with which to test theories of paleomonsoon behaviour. Here, we present an extension of the Liang Luar, Flores, speleothem δ18O record of past changes in southern hemisphere summer monsoon intensity back to 55.7 kyr BP. Negative δ18O excursions (stronger monsoon) occur during Heinrich events whereas positive excursions (weaker monsoon) occur during Dansgaard-Oeschger interstadials—a first order antiphase relationship with northern hemisphere summer monsoon records. An association of negative δ18O excursions with speleothem growth phases in Liang Luar suggests that these stronger monsoons are related to higher rainfall amounts. However, the response to millennial-scale variability is inconsistent, including a particularly weak response to Heinrich event 3. We suggest that additional drivers such as underlying orbital-scale variability and drip hydrology influence the δ18O response.

Antarctic Warming during Heinrich Stadial 1 in a Transient Isotope-Enabled Deglacial Simulation

Abstract Heinrich Stadial 1 (HS1) was the major climate event at the onset of the last deglaciation associated with rapid cooling in Greenland and lagged, slow warming in Antarctica. Although it is widely believed that temperature signals were triggered in the Northern Hemisphere and propagated southward associated with the Atlantic meridional overturning circulation (AMOC), understanding how these signals were able to cross the Antarctic Circumpolar Current (ACC) barrier and further warm up Antarctica has proven particularly challenging. In this study, we explore the physical processes that lead to the Antarctic warming during HS1 in a transient isotope-enabled deglacial simulation iTRACE, in which the interpolar phasing has been faithfully reproduced. We show that the increased meridional heat transport alone, first through the ocean and then through the atmosphere, can explain the Antarctic warming during the early stage of HS1 without notable changes in the strength and position of the Southern Hemisphere midlatitude westerlies. In particular, when a reduction of the AMOC causes ocean warming to the north of the ACC, increased southward ocean heat transport by mesoscale eddies is triggered by steeper isopycnals to warm up the ocean beyond the ACC, which further decreases the sea ice concentration and leads to more absorption of insolation. The increased atmospheric heat then releases to the Antarctic primarily by a strengthening zonal wavenumber-3 (ZW3) pattern. Sensitivity experiments further suggest that a ∼4°C warming caused by this mechanism superimposed on a comparable warming driven by the background atmospheric CO2 rise is able to explain the total simulated ∼8°C warming in the West Antarctica during HS1.

Millennial-scale glacier fluctuations on the southeastern Tibetan Plateau during MIS 2

The lack of high-resolution, well-dated records hinders our understanding of the climatic factors driving glacier fluctuations on the Tibetan Plateau. The Hengduan Ranges, on the southeastern Tibetan Plateau, are well suited to determining millennial-scale fluctuations in glacier extent. We investigated glacial landforms in the Mt. Gongga and Mt. Siguniang areas and obtained 46 10Be exposure ages. Age peaks and the maximum age reveal two glacier advances on Mt. Gongga (11.26±0.57–11.30±0.92ka, and 17.34±1.38–17.68±1.44ka); and three glacier advances on Mt. Siguniang (16.44±1.87–18.17±1.39ka, 20.48±1.90–22.72±1.74ka, and 29.98±3.96–30.77±2.36ka). We also analyzed a compilation of 181 recalculated 10Be exposure ages from the Hengduan Ranges. Glacier advances roughly correspond to four millennial-scale cold events (the Younger Dryas, and Heinrich events H1, H2 and H3) and are widely distributed in the Hengduan Ranges, which suggests that cooling, rather than precipitation change, was the main driver of glacier advances on the millennial-scale. Variations in monsoonal precipitation, which regulated the amplitude of glacier advances, were indicated by the depression of the equilibrium-line altitude.

Terrestrial records of deglaciation events during terminations V and IV in the central Apennines (Italy) and insights on deglacial mechanisms

40Ar/39Ar geochronology constraints to aggradational phases and grain size variations show that the two large gravel beds occurring in the sedimentary filling of the Liri fluvial-lacustrine basin (central Italy) recorded the occurrence of deglaciation events synchronous within uncertainties with global meltwater pulses at ca. 450 and 350 ka. In particular, we find a precise match between the ages of gravel deposition and the occurrence of moderate sea-level rise events which anticipate those more marked during the glacial termination V and IV in the Red Sea relative sea level curve, as already verified by data from the Tiber River catchment basin. Such correspondence suggests that gravel deposition is facilitated by melting of Apennine mountain range glaciers, which provide the water transport energy and a surplus of clastic input to the rivers draining the mountain regions and flowing into the Tyrrhenian Sea. Therefore, the thick gravel beds intercalated in the sedimentary filling of the catchment basins of the major rivers in central Italy may be regarded as an equivalent proxy of large deglaciation events, similar to the ice-rafted debris in northern Atlantic. Consistent with this hypothesis, we also show the close correspondence between the occurrence of particularly mild (warmer) minima of the mean summer insolation at 65° N and these early aggradational phases, as well as with other anomalous early sea-level rises occurring c. 750 ka and 540 ka at the onset of glacial termination VIII and VI, and 40 ka at the onset of the so-called Heinrich events.

Multi-phased Asian hydroclimate variability during Heinrich Stadial 5

Multi-phased Asian hydroclimate variability during Heinrich Stadial 5

Spatial pattern of orbital-to millennial-scale East Asian stalagmite δ18O variations during MIS 3

The Asian hydroclimate variability during the early and middle Marine Isotope Stage (MIS) 3 was reconstructed from a high-resolution stalagmite δ18O record from Qixing Cave in southern China, which exhibited an extraordinarily strong precessional cycle. When compared with other cave records (mostly) from East Asia, covering a latitudinal transect from 23° to 38°N, it can be found that the magnitude of precessional-scale stalagmite δ18O variations decreases when the latitude increases, approximately from 3‰ to 1‰. In contrast, the amplitude of Heinrich stadials (HSs) in these stalagmite δ18O records enhances with increasing latitudes, from 1‰ to 2‰, reflecting an alleviated effect of high-latitude climate changes on the low-latitude hydroclimate, or a relative contribution from Westerlies-borne and near-sourced 18O-enriched moisture at northern China. In southern China, orbital-scale changes in stalagmite δ18O closely match variations of the atmospheric CH4 concentration and δ18O of the atmospheric O2, suggesting a coherent response of low-latitude ecological and hydrological processes. In addition, a weaker precessional cycle in northern China is possibly related to the relative supply of near-sourced moisture source, where the Westerly wind can exert a substantial role. Therefore, precessional to millennial-scale changes in East Asian stalagmite δ18O records could be further modulated by local processes, rendering them a spatially contrasting trait. A pervasive imprint of muted precessional rainfall changes in loess records from northern China suggests a regionally distinct rhythm of moisture availability, in agreement with the stalagmite records from higher latitudes.

Coccolithophore paleoproductivity since the Last Glacial Maximum in the Atlantic Ocean: Relationship with calcification and preservation variability

Paleoproductivity, calcification and preservation patterns are presented from calcareous nannoplankton paleo-assemblages records covering the last ∼25 kyr retrieved from Integrated Ocean Drilling Program Sites U1385 (37°34.285′N; 10°7.562′W) and U1313 (41°0.0′N; 32°57.4′W), located respectively along the Western Iberian Margin and in the North Atlantic Ocean. The main aim of the study is to provide paleoceanographic and paleoclimatic analyses together with an investigation of changes in coccolithophore calcification processes occurring in response to climate variability from the Last Glacial Maximum up to the Holocene. Coccolithophore calcification was investigated by comparing variations in coccolith thickness and total coccolith calcite; at the same time dissolution/preservation indices were used to better understand the response of coccolithophores to changes in environmental parameters.In general, the increasing CO2 recorded during the last 25 kyr has a negative effect on calcification, resulting in a reduction of the different species size-normalized thicknesses at both sites. However, differences between the responses at the two latitudes studied are identified, in particular during the Heinrich Stadials; in contrast, during both Younger Dryas and Holocene intervals, the different size-normalized species thicknesses show consistent patterns at the two sites. During the Last Glacial Maximum the influence of subtropical waters due to Azores Current recirculation leads to a moderate productivity, initially at Site U1385 and later at Site U1313. At the same time, temperature influences the calcification at both sites. At Site U1385, the temperature is the main parameter affecting the paleoproductivity during Heinrich Stadial events; in addition, during Heinrich Stadial 1 alone, it also affects the calcification. At Site U1313 the calcification processes during Heinrich Stadial events are apparently hampered by an increasing water turbidity, caused by icebergs acting as ice rafted debris transporters, which affects light filtration in the water column. Light availability is an important calcification-influencing factor. At Site U1313 during Heinrich Stadial 1 and at the Last Glacial Maximum inception, dissolution, a consequence of increased CO2 solubility in the colder ice-age ocean, has a negative influence on calcification. Furthermore, at Site U1313, in contrast to Site U1385, the continually increasing CO2 concentration recorded during the Bølling-Allerød hampers both calcification and preservation.

Heinrich events and tectonic uplift as possible drivers for late Quaternary fluvial dynamics in the western Peruvian Andes

Late Quaternary fluvial valley development in Peru is considered to be driven by summer insolation maxima in the precession cycle, which coincide with increased precipitation and rising lake levels in the central Andes. Our literature review, however, indicates that fluvial aggradation does not always coincide with summer insolation maxima but may also occur during transitions between insolation maxima and minima or even during summer insolation minima. Tectonic uplift as a driver of fluvial incision has not been considered in the current terrace formation models, despite the Peruvian margin being located above an active subduction zone. We present new chronologic, stratigraphic and geomorphic data of the Cañete River valley over the past 102 ± 6 ka. We mapped its fluvial terraces (11.5°-13° southern latitude) along a 75-km long reach perpendicular to the strike of the Andes. Five fluvial terraces and one floodplain level were identified with relative elevations of up to 181.1 m above the floodplain. Thirty-three (pIR) IRSL ages of eight fluvial terraces and one alluvial fan sediment sample showed that their ages in part correspond to summer insolation maxima of the precession cycle, but that a better match exists with pluvial periods that coincided with Heinrich events of the northern hemisphere and rising lake levels in the Andes. The chronology of terraces of the Cañete River agrees with those of previously studied fluvial systems between 7° and 16° southern latitude in Peru, suggesting a regional-scale fluvial response possibly to the Heinrich events. Reconstructed longitudinal profiles and terrace ages were utilised to calculate vertical incision rates. We calculated a mean rate of 1.8 ± 0.10 mm a −1 over the last 102 ± 6 ka, but incision rates varied considerably within this time period possibly in response to changes in the Qs: Qw ratio as a consequence of increased sediment input during the wet Heinrich events. A set of 1338 GNSS measurements of daily, vertical crustal deformation of the years 2009–2015 showed a continuous and positive trend in interseismic, accumulative vertical crustal movements with a time-averaged, crustal uplift rate of 1.9 ± 3.6 mm a −1 . The positive trend in vertical movements, together with data from literature suggest landscape rejuvenation along the forearc and western Cordillera in central Peru. It is proposed that maximally ∼0.5 mm a −1 of the total incision over the past 102 ka may possibly be related to nonrecoverable, interseismic deformation. Climate change-driven variations in the Qs : Qw ratio are superimposed on the long-term tectonic uplift trend, and both are considered the main drivers of fluvial incision. • Fluvial aggradation in response to wet Heinrich events. • Very rapid incision of 1.8 ± 0.1 mm a −1 since 102 ± 6 ka until the present. • GNSS measurements reveal crustal uplift rate of 1.9 ± 3.6 mm a −1 . • Crustal uplift main driver for fluvial incision and climate change as secondary driver. • The same pattern of aggradation and incision occurred along entire western Peru.

Interactions of the Westerlies and Asian Summer Monsoon Since the Last Deglaciation in the Northeastern Qinghai‐Tibet Plateau

AbstractThe Asian summer monsoon (ASM), controlled by tropical ocean‐atmosphere systems, and the westerlies, directly linked to high‐latitude climates, interact in mid‐latitude East Asia. Located at a climate junction, the northeastern Qinghai‐Tibetan Plateau (QTP) is a sensitive zone for the interaction of the westerlies and ASM. In the current work, we present a new loess data set from the northeastern QTP since the last deglaciation, including grain size, stable isotope (δ13Corg and δ18Ocarb), total organic carbon, total nitrogen, magnetic susceptibility, and mineral composition. The PMIP3‐CMIP5 multi‐model ensemble and TraCE‐21ka transient simulation are used synthetically to analyze climate mechanisms. All proxies suggest instable glacial climate and stable interglacial climate in the northeastern QTP. The coarser grain size during the last deglaciation compared with the Holocene reflects intensified westerlies controlled by the weakening of Atlantic Meridional Overturning Circulation and existing ice sheets. More positive δ18Ocarb values during abrupt cooling events [Heinrich event 1 (H1) and the Younger Dryas (YD)] indicate the effect of a weakened monsoon triggered by the low‐latitude solar insolation and sea surface temperature. Therefore, the anti‐phase pattern between the ASM and westerlies is the primary reason for differences in climate stability. Moreover, low temperature favors the predomination of C3 plants and the development of permafrost, leading to more negative δ13Corg values and fine grain size during the H1 and YD. Taken together, the anti‐phase relationship between the westerlies and ASM emphasizes the complicated dynamic characteristics of atmospheric systems in the mid‐latitudes and the challenges for future predictions.

Vegetation history of a Mexican Neotropical basin from the late MIS 6 to early MIS 3: The pollen record of Lake Chalco

A new detailed pollen record of the penultimate glacial–interglacial–glacial cycle corresponding to the late marine isotopic stages (MISs) 6–3 (146–35 ka ago) has been obtained from Lake Chalco, in the Basin of Mexico. This record provides an insight into vegetation dynamics and climate variability at the northern limit of the American tropics. The pollen record displays a high temporal resolution with a mean sampling interval of ∼230 years. The changes in plant composition were reconstructed by pollen analysis and differences in plant composition between distinct periods were established. The wettest cold stage, indicated by a combination of diverse open vegetation and mesophytic taxa correlates with the penultimate late glacial MIS 6 (146–130 ka), when Lake Chalco was a deep freshwater lake. The penultimate interglacial MIS 5e (126.5–123.6 ka) shows a dominance of Pinus forests, with tropical taxa suggesting warmer than present conditions; environmental proxies indicate drier conditions and that the lake transitioned into a saline water body. Open communities and Pinus and Quercus forests with mesophytic taxa and, during the stadial periods, MIS 5 d, 5c, and 5 b (125–90 ka), with tropical taxa, characterize the record of interstadials. Pinus-dominated forests characterize the landscape during MIS 5a. MIS 5c to MIS 3 showed lower and variable evenness and palynological richness, along with an increase in fire activity. During MIS 4 and 3, a trend towards drier conditions was also recorded with enhanced carbonate precipitation and proxies indicating saline conditions. Tree-line changes in response to climate change and taxa migration were documented. A comparison with long records from the Greenland ice cores, the marine Cariaco Basin and insolation parameters revealed that the pollen data and the Pinus pollen accumulation rates evidence the response of the vegetation to glacial and interglacial changes and to millennial-scale variability. Wetter conditions were inferred to exist during the Greenland interstadials, while drier conditions persisted during Heinrich events.

Stalagmite evidence of Last Glacial Maximum to early Holocene climate variability in southwestern Iran

ABSTRACTGlacial periods and their terminations are useful for assessing the full scale of natural climate variability in the diverse climate regions of West Asia (i.e. deserts, mountains, alluvial plains, coastal zones). In this study, we report the first stalagmite stable isotope (δ18O and δ13C) records from the southern Zagros Mountains in southwest Iran. The records partially span the period from the Last Glacial Maximum (LGM) to the early Holocene, 24.2–9.6 thousand years before the present. The southwest Iran stalagmite δ18O and δ13C records indicate that climate and environment in the area differed substantially between the LGM and early Holocene. High stalagmite δ13C values are found at the LGM (7‰ greater compared to the early Holocene), and are attributed to sparse vegetation and reduced soil bio‐productivity, and possibly a greater degree of prior calcite precipitation in the epikarst, as a result of a cold and dry climate. Stalagmite δ18O values are also high at the LGM (4‰ greater compared to the early Holocene), and are attributed to lower temperatures (larger water–calcite isotope fractionation) and higher δ18O values of the moisture sources (Mediterranean and Red Seas). Through the deglaciation, stalagmite δ18O, δ13C and/or growth features coincide with the North Atlantic Heinrich Stadial 1, the Bølling–Allerød warm period and the Younger Dryas cold event, supporting a relationship between southern Zagros climate and the North Atlantic millennial events.

Late Pleistocene to early Holocene ENSO-related hydroclimatic changes revealed by palynological records from the coastal Xixi Basin, eastern China

We report a palaeontological record for the last 37.0 cal kyr BP in the Xixi Basin, eastern China, to better understand the correlation between hydroclimatic change and vegetation history during the late Pleistocene to early Holocene. Principal component analysis based on the 25 main palynological taxa indicated that dominant taxa fall into three groups: conifers that preferred dry conditions on the left of leading principal component (PC) 1, ferns and wetland communities associated with a humid climate responding to the positive loading of PC1, and Quercus-dominated communities principally corresponding to PC2. Based on this analysis, hydroclimatic conditions were relatively warm/dry during the latter Marine Isotope Stage (MIS) 3 with a high abundance of conifers. This was followed by a generally cold/wet MIS 2 with expansion of ferns and herbs, and a warm/dry MIS 1 with more evergreen species. The sporadic occurrence of evergreen species and drier conditions since about 19.0 cal kyr BP coincides with the beginning of the last deglaciation. An increase in sedimentary accumulation rates and changes in vegetation components at the onset of the Holocene are likely related to maximum transgression. Wet conditions coincide with several cold intervals, including Heinrich events and the Last Glacial Maximum. The overall warm/dry and cold/wet climatic patterns agree well with the palaeoclimatic records elsewhere in the middle Yangtze region, showing similar hydroclimatic forcing mechanisms related to the migration of the Meiyu Front modulated by the El Niño-Southern Oscillation (ENSO) activities. Cold/wet conditions probably correspond to enhanced ENSO and reduced insolation while warm/dry conditions correspond to dampened ENSO and increased insolation. The extremely humid conditions that occur during cold events may result from the combined effects of the Atlantic Meridional Overturning Circulation and ENSO. These results expand our understanding of the monsoonal hydroclimate in eastern China during the late Quaternary period.

Climate evolution in the Adriatic Sea across the last deglaciation: A multiproxy approach combining biomarkers and calcareous plankton

A multiproxy study combining calcareous plankton assemblages (coccolithophore and planktonic foraminifera), terrestrial (n-alkanes) and marine (alkenones) biomarkers was carried out in a sediment core (ND14Q-AR2) from the South Adriatic Sea. The focus of the study is to investigate millennial-to-centennial scale climate variability in the Eastern Mediterranean during the last deglaciation, between 20 and 11 ka BP. The high-resolution reconstruction allows for the characterization of the impact of the Heinrich Stadial 1 (HS1, here identified between 17.1 and 14.9 ka BP) at a centennial/multi-decadal time scale resolution. Based on terrestrial proxies, the HS1 interval has a two-fold partition: HS1a (17.1–15.9 ka BP), characterized by decreasing temperatures and relative high humidity, and HS1b (15.9–14.9 ka BP), characterized by the coldest temperatures and drier conditions on land. Terrestrial proxies suggest changes in moisture availability on land and ice melting delivery from the Alps during HS1. This period is followed by the Bølling-Allerød record (B-A, 14.9–12.3 ka BP) indicating ameliorated climate conditions and distinct local hydrological signals, related to global melt events similar to the Melt Water Pulse-1A. Finally, the comparison of our results with other Northern Hemisphere climatic records shows a good correspondence between the temperature variations in the South Adriatic Sea and the Greenland ice core oxygen isotope record, highlighting the climatic response of the South Adriatic to global climate variations. Comparison with other Mediterranean paleoclimatic records suggests a two-steps reorganization of the ocean/atmospheric circulation during the HS1 in the mid- and low-latitudes during the HS1 interval, but also a connection with the tropical Northern Hemisphere climate.

Terrestrial evidence for ocean forcing of Heinrich events and subglacial hydrologic connectivity of the Laurentide Ice Sheet.

During the last glacial period, the Laurentide Ice Sheet (LIS) underwent episodes of rapid iceberg discharge, recorded in ocean sediments as “Heinrich events” (HEs). Two competing models attempt to describe the stimulus for HEs via either internal ice sheet oscillations or external ocean-climate system forcing. We present a terrestrial record of HEs from the northeastern LIS that strongly supports ocean-climate forcing. Subglacial carbonate precipitates from Baffin Island record episodes of subglacial melting coincident with the three most recent HEs, resulting from acceleration of nearby marine-terminating ice streams. Synchronized ice stream acceleration over Baffin Island and Hudson Strait is inconsistent with internal ice sheet oscillations alone and indicates a shared ocean-climate stimulus to coordinate these different glaciological systems. Isotopic compositions of these precipitates record widespread subglacial groundwater connectivity beneath the LIS. Extensive basal melting and flushing of these aquifers during the last HE may have been a harbinger for terminal deglaciation.

The evolution of seafloor environmental conditions in the southern Red Sea continental shelf during the last 30 ka

The southern Red Sea is affected by intense interactions between monsoon and sea level changes in a semi-closed environment. Still, the impacts on the wide shelves remain poorly documented. We investigate the seafloor environmental conditions on the southern Red Sea shelf over the last 30 ka through sedimentological (visual inspection, granulometry, bulk density), geochemical (organic carbon, calcium carbonate and nitrogen content) and micropaleontological (benthic foraminifera) observations in a previously dated sediment core (FA09) from the Farasan Islands Archipelago. Glacial sediments exhibit high density and low carbonate contents, whereas post-glacial sediments reflect increased biogenic productivity. Seafloor oxygenation was limited in most of the glacial interval, along with hypersaline conditions. During the maximum sea level lowstand, an enhanced presence of eutrophic-indicator benthic assemblages likely reflects efficient organic matter preservation induced by weak water ventilation. Our proxies combined with a previous surface-water productivity reconstruction reveals a coupling between organic matter flux to the seafloor and surface productivity levels in the southern Red Sea shelf, particularly pronounced during periods of enhanced summer monsoon. This pattern is often accompanied by a decrease in seafloor oxygenation. In contrast, well oxygenated and more oligotrophic seafloor conditions are recorded around the time of Heinrich Stadial 2, when previous work indicates a reduction in summer monsoon intensity and surface productivity levels. A drastic decrease of benthic foraminifera along with increased organic carbon contents in late glacial and late Holocene reflect oxygen-depleted seafloor conditions that we attribute to expansion of an intensified southern Red Sea oxygen minimum zone (OMZ).

Millennial variability of terrigenous transport to the central–southern Peruvian margin during the last deglaciation (18–13 kyr BP)

Abstract. Reconstructing precipitation and wind from the geological record could help researchers understand the potential changes in precipitation and wind dynamics in response to climate change in Peru. The last deglaciation offers natural experimental conditions to test the response of precipitation and wind dynamics to high-latitude forcing. While considerable research has been done to reconstruct precipitation variability during the last deglaciation in the Atlantic sector of South America, the Pacific sector of South America has received little attention. This work aims to fill this gap by reconstructing types of terrigenous transport to the central–southern Peruvian margin (12 and 14∘ S) during the last deglaciation (18–13 kyr BP). For this purpose, we used grain-size distribution in sediments of marine core M77/2-005-3 (Callao, 12∘ S) and core G14 (Pisco, 14∘ S). We analyzed end-members (EMs) to identify grain-size components and reconstruct potential sources and transport processes of terrigenous material across time. We identified four end-members for both Callao and Pisco sediments. In Callao, we propose that the changes in the contributions of EM4 (101 µm) and EM2 (58 µm) mainly reflect the hydrodynamic energy and diffuse sources, respectively, while the variations in EM3 (77 µm) and EM1 (11 µm) reflect changes in the eolian and fluvial inputs, respectively. In Pisco, where there are strong winds and an extensive coastal desert, changes in the contribution of EM1 (10 µm) reflect changes in river inputs, while EM2 (52 µm), EM3 (75 µm), and EM4 (94 µm) reflect an eolian origin. At millennial scale, our record shows an increase in the fluvial inputs during the last part of Heinrich Stadial 1 (∼16–14.7 kyr BP) at both locations. This increase was linked to higher precipitation in the Andes related to a reduction of the Atlantic Meridional Overturning Circulation and meltwater discharge in the North Atlantic. In contrast, during the Bølling–Allerød interstadial (∼14.7–13 kyr BP), there was an eolian input increase, associated with stronger winds and lower precipitation that indicate an expansion of the South Pacific Subtropical High. These conditions would correspond to a northern displacement of the Intertropical Convergence Zone–South Pacific Subtropical High system associated with a stronger Walker circulation. Our results suggest that variations in river discharge and changes in surface wind intensity in the western margin of South America during the last deglaciation were sensitive to Atlantic Meridional Overturning Circulation variations and the Walker circulation on millennial timescales. In the context of global warming, large-scale increases in precipitation and fluvial discharge in the Andes as a result of a declining Atlantic Meridional Overturning Circulation and southward displacement of the Intertropical Convergence Zone should be considered.

Coupled atmosphere-ice-ocean dynamics during Heinrich Stadial 2

Our understanding of climate dynamics during millennial-scale events is incomplete, partially due to the lack of their precise phase analyses under various boundary conditions. Here we present nine speleothem oxygen-isotope records from mid-to-low-latitude monsoon regimes with sub-centennial age precision and multi-annual resolution, spanning the Heinrich Stadial 2 (HS2) — a millennial-scale event that occurred at the Last Glacial Maximum. Our data suggests that the Greenland and Antarctic ice-core chronologies require +320- and +400-year adjustments, respectively, supported by extant volcanic evidence and radiocarbon ages. Our chronological framework shows a synchronous HS2 onset globally. Our records precisely characterize a centennial-scale abrupt “tropical atmospheric seesaw” superimposed on the conventional “bipolar seesaw” at the beginning of HS2, implying a unique response/feedback from low-latitude hydroclimate. Together with our observation of an early South American monsoon shift at the HS2 termination, we suggest a more active role of low-latitude hydroclimate dynamics underlying millennial events than previously thought.