North Atlantic Heinrich Events Research Articles

Monsoon-related changes in surface hydrography and productivity in the Bay of Bengal over the last 45 kyr BP

Temporal variations of mixed-layer planktic foraminiferal abundances in a sediment core from the western Bay of Bengal (off Krishna – Godavari Basin) are used to reconstruct surface hydrographic structure and productivity in response to the changes in seasonal monsoon precipitation and wind intensity over the last 45 kyr BP. The faunal proxy records (abundances of eutrophic and oligotrophic species/group and G. bulloides / G. ruber ratio) exhibit millennial-scale variations during the late Glacial, deglacial and Holocene periods. Our results show that the surface hydrographic conditions and productivity varied in concert with the Northern Hemisphere climatic events (Dansgaard-Oeschger (D O) oscillations and Heinrich (H) events). The productivity in the late Glacial period between 44 and 19 kyr BP in general was higher compared to the deglacial and Holocene. Within the late Glacial period, we record high productivity during the cold phases of the D O cycles (43–41, 35–34, 28.5–28, 27–25 kyr BP), North Atlantic Heinrich events (39.5–38.5 kyr BP, H4; 31.5–30 kyr BP, H3; 24–23 kyr BP, H2) and during the Last Glacial Maximum (22–19 kyr BP). These productivity maxima are related to the pronounced advection of nutrient-rich subsurface waters to the euphotic zone induced by the intensified Northeast (NE) Monsoon winds in non-stratified/weakly stratified surface ocean conditions due to reduced Southwest (SW) Monsoon precipitation and freshwater influx. Our faunal record suggests a major reorganization of seasonal monsoon wind circulations after 16 kyr BP, when the SW monsoon intensified and concomitantly the NE monsoon weakened. Enhanced SW monsoon precipitation and freshwater flux to the Bay of Bengal between 15 and 13 kyr BP, a period broadly corresponding to the warm Bølling-Allerød (B/A) event, resulted in strong stratification of mixed-layer water and oligotrophic conditions. An increase in productivity between 13 and 11 kyr BP (equivalent to the cold Younger Dryas (YD) event) is attributed to intensified mixing of weakly stratified mixed layer waters by NE monsoon winds. During the early Holocene between 9 and 6 kyr BP, intensified SW monsoon precipitation and high fluvial discharge resulted in strongly stratified, nutrient-poor waters in the mixed-layer and a conspicuous reduction in productivity. However, a moderate increase in productivity is recorded between 5 and 3 kyr BP, when the SW monsoon intensity and associated fluvial discharge gradually declined and ocean surface stratification weakened resulting in conditions favourable for NE monsoon winds induced mixing. Overall, our results show that the alternating periods of intensified SW monsoon precipitation and NE monsoon wind circulation modulated the surface hydrographic structure and productivity patterns in the Bay of Bengal. • A 45 kyr planktic foraminiferal assemblage record from the western BoB. • Millennial scale variations in surface hydrography and productivity. • Seasonal monsoon precipitation & wind intensity modulate productivity pattern. • The productivity variation pattern is characterized by D O oscillations. • Productivity maxima occurred during cold stadials.

Benthic Foraminiferal Response to the Millennial-Scale Variations in Monsoon-Driven Productivity and Deep-Water Oxygenation in the Western Bay of Bengal During the Last 45 ka

In this study, we presented a high-resolution benthic foraminiferal assemblage record from the western Bay of Bengal (BoB) (off Krishna–Godavari Basin) showing millennial-scale variations during the last 45 ka. We studied temporal variations in benthic foraminiferal assemblages (relative abundances of ecologically sensitive groups/species, microhabitat categories, and morphogroups) to infer past changes in sea bottom environment and to understand how monsoon induced primary productivity-driven organic matter export flux and externally sourced deep-water masses impacted the deep-sea environment at the core site. Our records reveal a strong coupling between surface productivity and benthic environment on glacial/interglacial and millennial scale in concert with Northern Hemisphere climate events. Faunal data suggest a relatively oxic environment when the organic matter flux to the sea floor was low due to low primary production during intensified summer monsoon attributing surface water stratification and less nutrient availability in the mixed layer. Furthermore, records of oxygen-sensitive benthic taxa (low-oxygen vs. high-oxygen benthics) indicate that changes in deep-water circulation combined with the primary productivity-driven organic matter flux modulated the sea bottom oxygen condition over the last 45 ka. We suggest that the bottom water at the core site was well-ventilated during the Holocene (except for the period since 3 ka) compared with the late glacial period. At the millennial timescale, our faunal proxy records suggest relatively oxygen-poor condition at the sea floor during the intervals corresponding to the cold stadials and North Atlantic Heinrich events (H1, H2, H3, and H4) compared with the Dansgaard/Oeschger (D-O) warm interstadials. The study further reveals oxygen-poor bottom waters during the last glacial maximum (LGM, 19–22 ka) which is more pronounced during 21–22 ka. A major shift in sea bottom condition from an oxygenated bottom water during the warm Bølling–Allerød (B/A) (between 13 and 15 ka) to the oxygen-depleted condition during the cold Younger Dryas (YD) period (between 10.5 and 13 ka) is noticed. It is likely that the enhanced inflow of North Atlantic Deep Water (NADW) to BoB would have ventilated bottom waters at the core site during the Holocene, B/A event, and probably during the D-O interstadials of marine isotope stage (MIS) 3.

Massive deposition of Sahelian dust on the Canary Island Lanzarote during North Atlantic Heinrich Events

Abstract New IRSL ages of eolianites close to Muñique (Lanzarote) demonstrate the influence of millennial scale climatic variability on the sedimentary dynamics on the Canary Islands during the last glacial cycle. The repetitive succession of interstadial and stadial climatic conditions formed multiple sequences of eolian deposits, each in general comprising three depositional types. DepoType 1 and DepoType 2 consist mainly of marine biogenic carbonate detritus with small amounts of dust from the Sahara representing interstadial conditions. DepoType 2 compared to DepoType 1 is characterized by larger amounts of land snails and calcified brood cells. A DepoType 3 rich in dust from African subtropical/tropical Latisols terminates a sequence. IRSL dating on DepoType 3 type deposits clearly shows that these were deposited during Heinrich Events under stadial conditions. The stadial cooling of the North Atlantic Ocean caused a southern shift of climate zones that culminated during Heinrich Events when the arctic climate reaches its most southerly extent. As a consequence, atmospheric changes led to massive dust supply from the then-dry Sahel. The increase in dust and precipitation from the dry DepoTypes 1 to the more humid DepoTypes 3 originates from a modified atmospheric dynamic during a millennial cycle.

The role of Northeast Pacific meltwater events in deglacial climate change.

Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bølling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.

An ~130 kyr Record of Surface Water Temperature and δ18O From the Northern Bay of Bengal: Investigating the Linkage Between Heinrich Events and Weak Monsoon Intervals in Asia

AbstractMillennial‐scale reductions in monsoon precipitation, so‐called Weak Monsoon Intervals (WMIs), have been identified in numerous paleoclimate records across the Afro‐Asian monsoon domain throughout the last glacial‐interglacial cycle. These are considered the regional response to cooling during Heinrich events in the North Atlantic realm and several mechanisms have been suggested to explain this hemisphere‐scale climatic teleconnection. In particular, reductions in Indian Ocean sea surface temperature (SST) have been proposed as the linking element between Heinrich events and WMIs. However, the validity of this relationship has only been demonstrated for the last ~20 kyr, leaving unresolved whether it also holds on longer time scales. Here we present a new paired record of planktonic foraminifera‐based δ18Osw‐ivc and UK'37‐based SST from the northern Bay of Bengal, covering the last ~130 kyr. The δ18Osw‐ivc record clearly reflects orbitally paced changes of Indian Summer Monsoon intensity superimposed by centennial‐ to millennial‐scale WMIs that occurred synchronously to North Atlantic Heinrich events. Comparison with the UK'37‐based SST reconstruction reveals, however, that WMIs in most cases were not paralleled by ocean surface cooling, questioning whether Indian Ocean SST lowering was the linking element between Heinrich events and reductions in monsoon precipitation in Asia also during the last glacial period.

Spatio-temporal evolution of Australasian monsoon hydroclimate over the last 40,000 years

Maritime Continent (MC) convection drives zonal and meridional climate modes, including the Australasian monsoon system, but its role in past global climate change remains uncertain. Here we use speleothem oxygen isotope (δ18O) records from a latitudinal transect across the MC, and a new critically located record for southwest Sulawesi, Indonesia, to reconstruct spatial hydroclimate variability in the Australasian monsoon domain over the last 40,000 yr. Our results show that atmospheric convection in the core MC region was reduced from ∼40,000 to 12,000 yr ago and strengthened rapidly only after the inundation of shallow continental shelves in the region, well after the onset of deglaciation. Interestingly, the millennial-scale climatic impacts of North Atlantic Heinrich events are not evident in Sulawesi. Together with climate model simulations, we demonstrate that changes in deep atmospheric convection dominate glacial–interglacial hydroclimate over the MC, whereas latitudinal shifts in the mean location of the InterTropical Convergence Zone control rainfall patterns during abrupt climate changes.

Environmental change and human activity in the northeastern part of the North China Plain during early MIS-2

Marine isotope stage 2 (MIS-2), the coldest and the longest glacial during the last glacial period, was an important interval for the development and evolution of microlithic cultures. The Zhaocun site, located in the northeastern part of the North China Plain, is not only the first Late Paleolithic site discovered in the southern piedmont of the Yan Mountains, but also an important representative of Late Pleistocene strata and fossil fauna in North China. However, there have been few studies of the upper cultural layer at the site which have focused on the development of Paleolithic culture, especially the microlithic technology, in the context of environmental changes; moreover, no absolute agedeterminations are available. Here we present a study of the paleoenvironmental record of the Zhibuchang profile at the Zhaocun site during the interval of 27.6–23.6 cal. kyr BP. We conducted a stratigraphic study based on pollen and grain-size analyses of 78 samples from the profile, and obtained 7 AMS 14C dates. During 27.6–26.6 cal. kyr BP, the regional vegetation was relatively diverse meadow steppe dominated by Artemisia and Asteraceae. The climate, although colder and drier than today, was the warmest record within the studied profile, and was relatively favorable for human beings. From 26.6 cal. kyr BP, with the onset of the Last Glacial Maximum, the climate became colder and more variable, with two extreme climatic events occurred at 26.2–25.5 cal. kyr BP and 24.3–23.6 cal. kyr BP, respectively. A cold wet event from 26.2 to 25.5 cal. kyr BP led to a drastic decline in local biodiversity, which would have resulted in food shortages for human beings. In addition, previous areas available for human activities were gradually flooded due to the rising river level, forcing the relocation of human beings and resulting in the termination of the cultural layer. A cold dry event from 24.3 to 23.6 cal. kyr BP, which may correspond to North Atlantic Heinrich event 2, caused a further deterioration of the environment for human beings, and may be a major cause of the absence of microlithiccultural remains on the northeastern partofthe North China Plain during late MIS-2.

Moisture availability in the southwest United States over the last three glacial-interglacial cycles.

The projected long-term drying of the southwest (SW) United States in response to climate warming raises a sobering alarm for this already water-limited region, yet the climatic controls on moisture availability over longer time scales remain a topic of debate. Here, we present a 350,000-year record of past water table fluctuations in Devils Hole 2 cave that are driven by variations in recharge amount to the local groundwater flow system. Because of the unprecedented length and precision of our record, we can observe variations in regional moisture availability over the last three glacial-interglacial cycles at a millennial-scale resolution. The timing of past water table rises and falls (>9 m in amplitude) closely coincides with the expansion and reduction of Northern Hemisphere ice volume, which in turn influences the position and intensity of westerly winter storms on orbital time scales. Superimposed on this long-term trend are millennial-scale highstands recorded during the last glaciation that coincide with North Atlantic Heinrich events. Earlier millennial-scale highstands provide the first evidence of multiple short-lived wet periods in the SW United States linked to coeval cooling intervals in the North Atlantic during marine isotope stages 6 and 8. The Devils Hole 2 water table record is currently the longest independently dated paleomoisture record in the SW United States and thus provides a critical testbed to examine the controls on regional moisture availability over larger time scales.

Valley glaciers persisted in the Lake District, north‐west England, until ∼16–15 ka as revealed by terrestrial cosmogenic nuclide (10Be) dating: a response to Heinrich event 1?

ABSTRACTThe Lake District of north‐west England acted as an independent centre of ice dispersal within the more extensive British–Irish Ice Sheet (BIIS) during the Last Glacial Maximum, but relatively little is known about the pattern and timing of glacier retreat. Four new terrestrial cosmogenic nuclide (10Be) surface exposure ages from boulders from a lateral moraine in the Duddon valley, south‐west Lake District, have yielded internally consistent ages with uncertainty‐weighted means of 16.51 ± 0.78 ka (using the Loch Lomond production rate with Lm scaling and 1 mm ka−1 erosion rate) and 16.15 ± 1.30 ka (using CRONUScalc with SA scaling and 1 mm ka−1 erosion rate). It is inferred that glacier retreat from the moraine occurred in the interval ∼16.5–16.1 ka but that a valley glacier continued to exist, probably until ∼15 ka. The Duddon valley ages agree with other surface exposure ages from Wasdale, Watendlath and the Shap fells, together demonstrating that glacier ice was still widespread in the Lake District at ∼17–15 ka. There is also consistency with ages from other sectors of the BIIS that are considered to have responded to North Atlantic Heinrich event 1.

Millennial changes in North Atlantic oxygen concentrations

Abstract. Glacial–interglacial changes in bottom water oxygen concentrations [O2] in the deep northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O2] changes in relation to millennial climate oscillations in the North Atlantic over the last glacial cycle, using bottom water [O2] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015) and (2) ODP (Ocean Drilling Program) Site 1055 from the intermediate northwest Atlantic. The deep northeast Atlantic core MD95-2042 shows decreased bottom water [O2] during millennial-scale cool events, with lowest bottom water [O2] of 170, 144, and 166 ± 17 µmol kg−1 during Heinrich ice rafting events H6, H4, and H1. Importantly, at intermediate depth core ODP Site 1055, bottom water [O2] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with the lowest values of 179 and 194 µmol kg−1 recorded during millennial cool event C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O2] across the North Atlantic below ∼ 1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O2] during North Atlantic Heinrich events and earlier cold events at the two sites can be linked to water mass changes in relation to ocean circulation changes and possibly productivity changes. At the intermediate depth site a possible strong North Atlantic Intermediate Water cell would preclude water mass changes as a cause for decreased bottom water [O2]. Instead, we propose that the lower bottom [O2] there can be linked to productivity changes through increased export of organic material from the surface ocean and its subsequent remineralization in the water column and the sediment.

North Atlantic climatic changes reflected in the Late Quaternary foraminiferal abundance record of the Andaman Sea, north-eastern Indian Ocean

Here, we present planktonic foraminiferal, benthic foraminiferal and other proxy profiles from well-dated sediment cores in the Andaman Sea showing changes in climate and oceanography at Dansgaard-Oeschger (D/O) and Heinrich scales. The large temporal variations in the abundances of total benthic foraminifera, Globigerina rubescens and Neogloboquadrina dutertrei suggest substantial changes in the surface to bottom hydrography of the Andaman Sea. G. rubescens abundance minima during the last glacial cycle correspond to warm interstadials (D/O events 1 to 14) while maxima correspond to Heinrich events (H1 to H4), the last glacial maximum (LGM) and the Younger Dryas. D/O events are marked by very low G. rubescens and high N. dutertrei abundances which indicate freshened surface water related to increased direct precipitation (over evaporation) and strengthened Irrawaddy outflow. Lower abundance of N. dutertrei (and higher abundance of G. rubescens) during North Atlantic Heinrich events, the deglacial, the YD and the mid- to late-Holocene reflect reduced influx of fresh water as a result of weakened summer monsoon freshwater input. The timing of these Andaman Sea monsoonal changes indicate a strong teleconnection to North Atlantic climate change.

Sortable silt cycles in Svalbard slope sediments 74–0 ka

ABSTRACTThe grain size of sortable silt (SS, 10–63 μm) was studied in sediment core JM04‐025PC from a sediment drift on the lower part of the western Svalbard slope. The study aims to reconstruct the bottom current activity and intermediate water circulation in the Greenland Sea. The SS record covers the last 74 ka and shows consistent millennial‐scale oscillations in grain size. The youngest part of the record (<25 ka) is well correlated with the few available previously published reconstructions of grain size from the North Atlantic Ocean and the Nordic seas. Fining of SS occurs during four well‐known regional cold climatic intervals, the 8.2‐ka cold spell, the Younger Dryas and North Atlantic Heinrich Events H1 and H2, as well as during a local meltwater event at 14.4–14.7 ka, possibly coinciding with the Older Dryas. Also before 25 ka, grain size oscillated in pace with the Northern Hemisphere climate with finer grain sizes during cold periods and coarser grain sizes during warm periods. The fining of SS indicates reduced bottom current strength and reduced water exchange between the Greenland Sea and the Arctic Ocean during regional climate cooling events, and probably also reduced deep ocean convection in the Greenland Sea gyre.

North Atlantic marine radiocarbon reservoir ages through Heinrich event H4: a new method for marine age model construction

AbstractCooling and sinking of dense saline water in the Norwegian–Greenland Sea is essential for the formation of North Atlantic Deep Water. The convection in the Norwegian–Greenland Sea allows for a northward flow of warm surface water and southward transport of cold saline water. This circulation system is highly sensitive to climate change and has been shown to operate in different modes. In ice cores the last glacial period is characterized by millennial-scale Dansgaard–Oeschger (D–O) events of warm interstadials and cold stadials. Similar millennial-scale variability (linked to D–O events) is evident from oceanic cores, suggesting a strong coupling of the atmospheric and oceanic circulations system. Particularly long-lasting cold stadials correlate with North Atlantic Heinrich events, where icebergs released from the continents caused a spread of meltwater over the northern North Atlantic and Nordic seas. The meltwater layer is believed to have caused a stop or near-stop in the deep convection, leading to cold climate. The spreading of meltwater and changes in oceanic circulation have a large influence on the carbon exchange between atmosphere and the deep ocean and lead to profound changes in the 14C activity of the surface ocean. Here we demonstrate marine 14C reservoir ages (R) of up to c. 2000 years for Heinrich event H4. Our R estimates are based on a new method for age model construction using identified tephra layers and tie-points based on abrupt interstadial warmings.

Fluctuations in the late Quaternary East Asian winter monsoon recorded in sediment records of surface water cooling in the northern Japan Sea

AbstractThe East Asian monsoon climate system involves interactions between land and ocean and is composed of summer and winter monsoons. Cooling of surface water by the East Asian winter monsoon (EAWM) promotes the formation of sea ice and deep water in the northern Japan Sea. Modern observations suggest close relationships between intensification of the EAWM and both sea ice formation and sea‐surface temperature (SST) decrease in the northeastern Japan Sea. Records of ice‐rafted debris (IRD) and alkenone‐based SST in the northeastern Japan Sea are examined to clarify the temporal variability of the EAWM and its controlling factors. Close relationships with oxygen isotope records of Chinese stalagmites and Northern Hemisphere summer insolation are observed in the records of IRD occurrence and SST during Marine Isotope Stages (MIS) 3–5. The co‐occurrence of maxima in the abundance of IRD and minima in SST in the northeastern Japan Sea indicates that the intensity of the EAWM was enhanced during periods of low summer insolation and high oxygen isotope ratios in Chinese stalagmites. Decoupling between the IRD and SST records during MIS 2 and MIS 6 might be the result of differences in surface water conditions caused by low sea levels and the resultant closure of the Tsushima Strait. During MIS 3 and 4, some small but clear maxima in IRD and minima in SST occurred at relatively low summer insolation periods. Some of these peaks co‐occur with North Atlantic Heinrich events, suggesting a possible climate linkage between the North Atlantic and the Asian monsoon. The presence of finer fluctuations in the occurrence of IRD than in summer insolation suggests finer millennial–centurial timescale fluctuations in the intensity of the EAWM. Furthermore, some IRD peaks occurred during periods of high insolation, such as 39, 86, 103, 128, 145, and 153 ka. Some of these IRD peaks correlate with minima in reconstructed SST and maxima in oxygen isotope ratios of Chinese stalagmites. This result suggests that extremely cold winters occur even during periods of enhanced East Asian summer monsoon intensity, and indicates that solar insolation is not the only factor that controls variations in the intensity of the EAWM at millennial–centurial timescales. Copyright © 2012 John Wiley & Sons, Ltd.

Late Quaternary sedimentological and climate changes at Lake Bosumtwi Ghana: New constraints from laminae analysis and radiocarbon age modeling

The Lake Bosumtwi sediment record represents one of the longest and highest-resolution terrestrial records of paleoclimate change available from sub-Saharan Africa. Here we report a new sediment age model framework for the last ~45cal kyr of sedimentation using a combination of high-resolution radiocarbon dating, Bayesian age-depth modeling and lamination counting. Our results highlight the practical limits of these methods for reducing age model uncertainties and suggest that even with very high sampling densities, radiocarbon uncertainties of at least a few hundred years are unavoidable. Age model uncertainties are smallest during the Holocene (205yr) and the glacial (360yr) but are large at the base of the record (1660yr), due to a combination of decreasing sample density, larger calibration uncertainties and increases in radiocarbon age scatter. For portions of the chronology older than ~35cal kyr, additional considerations, such as the use of a low-blank graphitization system and more rigorous sample pretreatment were necessary to generate a reliable age depth model because of the incorporation of small amounts of younger carbon. A comparison of radiocarbon age model results and lamination counts over the time interval ~15–30cal kyr agree with an overall discrepancy of ~10% and display similar changes in sedimentation rate, supporting the annual nature of sediment laminations in the early part of the record. Changes in sedimentation rates reconstructed from the age-depth model indicate that intervals of enhanced sediment delivery occurred at 16–19, 24 and 29–31cal kyr, broadly synchronous with reconstructed drought episodes elsewhere in northern West Africa and potentially, with changes in Atlantic meridional heat transport during North Atlantic Heinrich events. These data suggest that millennial-scale drought events in the West African monsoon region were latitudinally extensive, reaching within several hundred kilometers of the Guinea coast. This is inconsistent with a simple southward shift in the mean position of the monsoon rainbelt, and requires changes in moisture convergence as a result of either a reduction in the moisture content of the tropical rainbelt, decreased convection, or both.

The nature, timing and controls of the Quaternary development of the Rio Bergantes, Ebro basin, northeast Spain

Geomorphological, sedimentological and geochronological analyses have been undertaken to decipher the effects of Quaternary environmental change on the development of the Rio Bergantes, Ebro basin, northeast Spain. Field mapping and profiling using aerial photography and airborne LiDAR data have identified a sequence of six river terraces. Age control, provided by luminescence dating, has revealed two major phases of valley floor aggradation that appear to have occurred at ~183–130ka (Terrace BT1, elevated ~25m above the modern river channel) and ~111–26.8ka (BT2, ~18–20m above the modern river channel), punctuated by ~20m of incision in the intervening period. Following these two phases of large-scale aggradation, the Bergantes river system appears to have adjusted to a relatively sediment-limited regime and responded to environmental changes through a series of smaller cut and fill cycles. This resulted in four lower elevation terraces (BT3–BT6) that are inset within the older (BT1 and BT2) valley fills. These lower terraces have been temporally constrained to ~24.5–14.2ka (BT3); 10.6–9.8ka (BT4); 8.3–6.2ka (BT5) and ~3ka (BT6). Field sedimentological and stratigraphic analyses of the alluvial fills indicate the influence of tributary and hill slope derived sediment supply during the aggradation of BT1 and, to a lesser extent BT2. This contrasts with the predominantly trunk river derived deposits which make up the fill of BT3–BT6, when the hillslopes and tributaries in the study area appear to have been relatively de-coupled from the trunk river. Data presented in this paper suggest that the Bergantes river system has responded to, and has preserved, a record of environmental changes operating both in synchrony with, and at a higher frequency than, Milankovitch-driven glacial–interglacial cycles. Major net aggradation phases (those associated with relatively large amounts of fluvial sediment e.g. BT1 and BT2) appear to coincide with Pleistocene glacial conditions in MIS 6, and parts of MIS 5 and MIS 4, when the climate on the Iberian Peninsula was cool and dry. Intervening phases of net valley incision appear to coincide with warmer and moister climates in Pleistocene interglacials. The younger terraces (BT3–BT6) are characterised by much smaller amounts of fluvial sediment supply. Dating control places their development during stadial (aggradation) and interstadial (incision) events of the Late Pleistocene and Holocene. Some phases of aggradation appear to coincide with North Atlantic Heinrich Events, and also with other securely dated phases of aggradation elsewhere in the Mediterranean basin.

A ∼43-ka record of paleoenvironmental change in the Central American lowlands inferred from stable isotopes of lacustrine ostracods

We present a continuous ostracod isotope (δ18O and δ13C) record from Lake Petén Itzá, Petén, Guatemala, in the northern, lowland Neotropics that spans the last ∼43 cal ka BP. Variations in oxygen and carbon isotopes closely follow lithologic variations, which consist of alternating gypsum and clay deposits that were deposited under relatively dry and wet climate, respectively. During the last glacial period, the greatest δ18O and δ13C values coincide with gypsum deposited during lake lowstands under arid climate conditions that were correlated previously with North Atlantic Heinrich events. In contrast, interstadials and the entirety of the Last Glacial Maximum (∼24–19 cal ka BP) are marked by clay deposition and lower δ18O and δ13C values, reflecting higher lake levels and relatively moister climate.Isotope results and pollen data, along with independently inferred past water levels, show the early deglacial period (∼19–15 cal ka BP) was the time of greatest aridity and lowest lake stage of the past 43 ka. This period occurred during Heinrich Stadial 1 (HS 1), when an extensive tropical megadrought has been postulated (Stager et al., 2011). Heinrich Stadial 1 is represented by two episodes of gypsum precipitation and high δ18O and δ13C values in Petén Itzá, interrupted by an intervening period of lower δ18O and δ13C and clay deposition centered on ∼17 cal ka BP. The two periods of inferred maximum cold and/or arid conditions at ∼17.5 and 16.1 cal ka BP coincide approximately with two pulses of ice-rafted debris (IRD) recorded off southern Portugal (Bard et al., 2000). At ∼15 cal ka BP, coinciding with the start of the Bolling-Allerod period, δ18O and δ13C decrease and gypsum precipitation ceases, indicating a transition to warmer and/or wetter conditions. Gypsum precipitation resumed while δ18O and δ13C increased at the start of the Younger Dryas at 13.1 cal ka BP and continued until 10.4 cal ka BP, near the onset of the Holocene.Precipitation changes during the last glacial period in the northern hemisphere Neotropics were closely linked with freshwater forcing to the high-latitude North Atlantic, and sensitive to changes in the location of meltwater input. Climate was coldest/driest when meltwater directly entered the high-latitude North Atlantic, permitting sea ice expansion and weakening of Atlantic Meridional Overturning Circulation (AMOC), which resulted in a more southerly position of the Intertropical Convergence Zone (ITCZ). Upon deglaciation, when meltwater was directed to the Gulf of Mexico, at ∼17 ka and during the Bolling-Allerod period (15–13 ka), precipitation increased in the northern hemisphere Neotropics as North Atlantic sea ice retreated and the ITCZ shifted northward. Results from Lake Petén Itzá offer some support for the meltwater routing hypothesis of Clark et al. (2001).

Rapid climate change and variability of settlement patterns in Iberia during the Late Pleistocene

Due to its diverse geographic and climatic conditions, the Iberian Peninsula is well suited for studies into the relationship between climate, environment and hunter-gatherer adaptation. With focus on the archaeological record, this paper examines to what extent diachronic variations in site density on the Iberian Peninsula are related to climate variability and cultural change. Studies are based on a comprehensive record of technocomplexes that date from the late Middle Palaeolithic, early Upper Palaeolithic, Gravettian and Solutrean. The record comprises altogether 152 archaeological cave sites and rock shelters. Analysis reveals strong regional differences between Northern and Southern Iberia, both in isochronic and in diachronic perspective. This is expressed by the strongly different patterns of human presence in these regions. In particular, within both regions major cultural changes coincide with the environmental impact of North Atlantic Heinrich Events (HE). From previous studies, it is known that the human population on the Iberian Peninsula (IP) must have suffered strongly under the extremely variable climate conditions during the Late Pleistocene. Based on extensive site-mapping, the hypothesis is that during HE a major disintegration of habitats must have occurred, with various but strongly isolated patchy refugia remaining. Further, during HE, Southern Iberia could not uphold its previous function in providing a reliable refuge for humans. Not only does climatic deterioration during the different HE repeatedly lead to a near-complete breakdown of settlement patterns, but following each HE there is a major reorganization in settlement patterns on the IP.

Productivity collapses in the Arabian Sea during glacial cold phases

Productivity in the Arabian Sea is one of the highest in the world. It is controlled by seasonally reversing monsoonal wind‐driven upwelling of nutrient‐rich deeper waters which fuel phytoplankton growth. The detailed history of upwelling‐induced productivity in the eastern Arabian Sea is unknown. Here we present paleoproductivity records from a composite sediment core at the millennial scale during the last 80 kyr B.P. These records are based on relative abundance counts of planktonic foraminifera and organic carbon contents, which are shown to mainly vary in concert. The eastern Arabian Sea upwelling‐induced productivity was higher in the glacial period than in the Holocene, but it fell repeatedly on millennial timescales. These productivity declines occurred during cold events in the North Atlantic region, with the most pronounced changes prevailing during the Heinrich events. Hence, seasonal monsoon winds that drive upwelling‐induced productivity in the east were weak when the North Atlantic was cold. These weak winds resulted in stratification of the water column, comparable to today's Arabian Sea stratification in the intermonsoonal period. Combining the new eastern with published western Arabian Sea results shows that the entire biological factory was severely diminished during the North Atlantic Heinrich events, and the seasonal productivity change in the Arabian Sea monsoon system was reduced with year‐round low productivity.

Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350 kyr

We measured productivity proxies (chlorin, carbonate and organic carbon, opal, and biogenic Ba content) and lithophysical proxies (magnetic susceptibility, water content, density, and coarse sediment fraction) in sediment of central Okhotsk Sea core PC-7R. The age model covering the last 350kyr of this core was constructed by correlating the dated series of relative paleointensity lows recognized in geomagnetic intensity records, marine isotope stage (MIS) boundaries determined in broad variations of the lithophysical and productivity proxies, and tephrochronology. The orbital changes of the lithophysical and productivity proxy stacks lag behind Northern Hemisphere summer radiation by approximately 6.3 and 5.9kyr, respectively. This lag is consistent with a Milankovich model of climate control by solar radiation through the northern ice sheet volume and sea surface and surrounding land responses, which are fast compared with sedimentological evidence. Productivity proxies of the Okhotsk Sea also demonstrate 52 abrupt, pronounced productivity minima associated with regional climate coolings during the last 350kyr, which present useful indicators of millennial-scale climate changes in this marginal sea. Based on the postulated synchronicity of Dansgaard–Oeschger cycles in the Northern Hemisphere and the established simultaneity of 11 Okhotsk Sea coolings in the last 77kyr with abrupt severe cold events in the Greenland ice core and North Atlantic Heinrich events, all of these may be regarded as Heinrich-equivalent event anomalies. The Okhotsk Sea events have their counterparts in the records of North Atlantic sediments, the Greenland ice sheet, East Asia summer monsoon, and the Antarctic ice sheet. Probably the Arctic Oscillation was the main factor determining orbital and millennial climate oscillations in the high-latitude Northern Hemisphere, including the Okhotsk Sea region.