Millennial-scale Climate Fluctuations Research Articles

A Walker switch mechanism driving millennial-scale climate variability

<p>A key goal of paleoclimate science is to identify the source of millennial-scale climate fluctuations. Although long-term changes in the Earth��s position relative to the Sun are the driver of climate change on timescales of �� 19 thousand years (ka), it is still incompletely understood what causes rapid sub-orbital changes in climate that were common during glacial periods. Here, we provide a continuous, precisely dated speleothem record from southeastern Alaska that spans the last 13.5 ka. Despite its location in the high latitudes of the Northern Hemisphere, this record shows an equatorial Pacific climate pattern during the end of the last deglaciation and Holocene, and is at odds with North Atlantic climate patterns, calling into question the bipolar seesaw mechanism for the Pacific realm. Because of this, we propose a new mechanism to account for sub-orbitally forced climate changes. The mechanism, termed the Walker switch, is forced by insolation and results in rapid zonal sea-surface temperature changes in the equatorial Pacific. The climatic effects of the Walker switch are propagated across the globe, including the high northern latitudes.</p>

Extracting causation from millennial-scale climate fluctuations in the last 800 kyr

The detection of cause-effect relationships from the analysis of paleoclimatic records is a crucial step to disentangle the main mechanisms at work in the climate system. Here, we show that the approach based on the generalized Fluctuation–Dissipation Relation, complemented by the analysis of the Transfer Entropy, allows the causal links to be identified between temperature, CO_2 concentration and astronomical forcing during the glacial cycles of the last 800 kyr based on Antarctic ice core records. When considering the whole spectrum of time scales, the results of the analysis suggest that temperature drives CO_2 concentration, or that are both driven by the common astronomical forcing. However, considering only millennial-scale fluctuations, the results reveal the presence of more complex causal links, indicating that CO_2 variations contribute to driving the changes of temperature on such time scales. The results also evidence a slow temporal variability in the strength of the millennial-scale causal links between temperature and CO_2 concentration.

Relict proglacial lake of Spituk (Leh), northwest (NW) Himalaya: A repository of hydrological changes during Marine Isotopic Stage (MIS)-2

In the northwestern Himalaya (Ladakh and Karakoram), the spatial and temporal changes in the late Quaternary glaciation were modulated by a combination of two contrasting moisture sources (viz., the Indian Summer Monsoon and the mid-latitude westerlies). The inferences about their relative contribution to glacier expansion are based on the ages obtained on moraines. Since the moraines suffer from poor preservation and chronological uncertainty, the present study, therefore, resorted to a near-continuous relict proglacial lake succession in order to reconstruct the continuous pattern of glacier fluctuations during Marine Isotopic Stage-2 (MIS-2). Since the AMS radiocarbon ages (both organic and inorganic matter) suffered from various complexities such as dead carbon contribution and hard water effect, the paleohydrological inferences in this study were drawn primarily based on optical ages (along with sedimentological and geochemical analysis).The proglacial lake sedimentation was modulated by the temporal changes in meltwater discharge. This is attributed to the waning and waxing of proximal valley glaciers. Evidence accrued from the lake sediment supported by the published moraine chronology suggests that valley glaciers expanded during the onset of MIS-2 and persisted until around the Last Glacial Maximum (LGM). The beginning of MIS-2 is marked by the onset of cooling (~ 30 ka), which led to a marginal advance in the glaciers. Additionally, early MIS-2 was characterised by millennial-scale climatic fluctuations. For example, a short-lived increase in humidity was observed between 30.2 ka and 29.3 ka, followed by a reversal towards a drier climate between 29.3 ka and 28.1 ka. During the early part of MIS-2, glaciers were driven by the decrease in temperature, whereas the enhanced moisture contribution during mid MIS-2 was responsible for extensive glacier growth. We hypothesise that the enhanced moisture was contributed by the mid-latitude westerlies, with subordinate contribution from the Indian Summer Monsoon (ISM). This we attribute to the southward positioning of the Intertropical Convergence Zone (ITCZ) during MIS-2.

Paleoenvironments from robust loess stratigraphy using high-resolution color and grain-size data of the last glacial Krems-Wachtberg record (NE Austria)

The complex interplay of dust sedimentation, pedogenesis, and erosion/reworking in the formation of loess-paleosol sequences (LPS) challenges paleoenvironmental proxies. Here we show that color and grain size are essential parameters characterizing loess profiles and support robust stratigraphies as a basis for reconstructions in the context of local geo-ecological and large-scale paleoclimatic evolution. Detailed paleoenvironmental records from the period since the arrival of anatomically modern humans to the last glacial maximum are scarce in the Alpine surroundings. The c. 7.5 m thick LPS Krems-Wachtberg, NE Austria, known for its well-preserved Upper Paleolithic context at a depth of 5.5 m, formed between 40 and 20 ka BP by quasi-continuous dust-sedimentation, interrupted by phases of incipient pedogenesis and local reworking. The new KW2015 composite is based on three sections studied and sampled at 2.5 cm resolution. Color and grain size data support a robust stratigraphy for reconstructions of the pedosedimentary evolution. The marked transition from oxidized to reduced paleosols of KW2015 around 34–35 ka corresponds to the Middle-to Upper Pleniglacial transition as part of a general cooling trend from marine isotope stage (MIS) 3 to 2, intensely modulated by millennial-scale climatic fluctuations as recorded in the Greenland ice core data. The distinct response of KW2015 to these trends highlights that reconstructing LPS evolution based on a robust stratigraphy is a prerequisite to paleoenvironmental proxy interpretation.

Millennial-scale vegetation and climatic changes from an Early to Mid-Holocene lacustrine archive in Central Ganga Plains using multiple biotic proxies

The Indo-Gangetic floodplains have many abandoned meander cut-offs and oxbow lakes that can serve as potential archives for high-resolution paleoclimatic reconstruction. Previously, a few of these lakes have been investigated for understanding the regional Holocene climatic history, but these studies are generally limited by a lack of high chrono-stratigraphic resolution. To develop an understanding of the role of local and/or regional climate forcings, an oxbow lake (Baraila Tal) in the Central Ganga Plains has been studied using multiple biotic proxies such as pollen, phytoliths, diatoms, and sponge spicules. Millennial-scale climatic fluctuations and vegetation history for the Early to Mid-Holocene period have been reconstructed using 17 AMS radiocarbon dates from two trenches, 3–4 m deep. Five major biotic zones have been delineated between 13.2 and 4.8 kyr BP. Zone I (∼13–12 kyr BP) shows small patches of dry tropical woody flora along with tall C4 grass indicating a moderately dry climate. The presence of low-temperature herbs indicates an active NE monsoon but the paucity of arboreals indicates a weakened Indian Summer Monsoon (ISM). The high percentages of Phacotus lenticularis cysts in Zone II (12–9 kyr BP) indicate the presence of stagnant, lime-rich inland waters under relatively colder climatic conditions. The appearance of evergreen forest groves around 11.5 kyr BP indicates an increase in humidity, and lake deepening was inferred between 10.9 and 10.3 kyr BP. Sediment accumulation rate shows a sudden drop at ∼10.3 kyr BP indicating a complete cut-off of the oxbow lake from the fluvial channel. Zone III (9–8 kyr BP) records millennial-scale drying with a complete absence of aquatic pollen between 8.9 and 7.7 kyr BP. The expansion of short C4 grass in Zone IV (8–7 kyr BP) further indicates drier conditions. Zone V (7 –∼5 kyr BP) shows very high percentages of sponge spicules that flourish in clear waters, and represents a strengthened ISM and a subsequent lake expansion.

A taxonomic and taphonomic study of Pleistocene fossil deposits from the western Nefud Desert, Saudi Arabia

AbstractOver the past decade, a growing interest has developed on the archaeology, palaeontology, and palaeoenvironments of the Arabian Peninsula. It is now clear that hominins repeatedly dispersed into Arabia, notably during pluvial interglacial periods when much of the peninsula was characterised by a semiarid grassland environment. During the intervening glacial phases, however, grasslands were replaced with arid and hyperarid deserts. These millennial-scale climatic fluctuations have subjected bones and fossils to a dramatic suite of environmental conditions, affecting their fossilisation and preservation. Yet, as relatively few palaeontological assemblages have been reported from the Pleistocene of Arabia, our understanding of the preservational pathways that skeletal elements can take in these types of environments is lacking. Here, we report the first widespread taxonomic and taphonomic assessment of Arabian fossil deposits. Novel fossil fauna are described and overall the fauna are consistent with a well-watered semiarid grassland environment. Likewise, the taphonomic results suggest that bones were deposited under more humid conditions than present in the region today. However, fossils often exhibit significant attrition, obscuring and fragmenting most finds. These are likely tied to wind abrasion, insolation, and salt weathering following fossilisation and exhumation, processes particularly prevalent in desert environments.

Stratigraphic and geomorphologic evidence of three MIS 2 glacial advances in the South Fork Hoh River valley, Olympic Mountains, Washington, USA

AbstractThe geomorphology and stratigraphy of the South Fork Hoh River (SF Hoh), Olympic Mountains, Washington, allow for greater understanding of Marine Oxygen Isotope Stage 2 (MIS 2) ice fluctuations, glacial dynamics, and sedimentation. Age control from optically stimulated luminescence and radiocarbon dating constrains deposited sediments associated with four Late Pleistocene ice-marginal positions that formed under reduced ice volume conditions compared with MIS 3–5 glaciers in the same drainage. The earliest MIS 2 ice margin extended into the main Hoh River valley (pre–SF 1, 28 ka to >23.0 ka). After retreat, the ice occupied three closely spaced ice-marginal positions (SF 1–3) that range in age from 22.0 ka to shortly after 18.7 ka. While the SF 1 and SF 3 positions were previously identified as the Twin Creeks I and II positions, the intermediate SF 2 position had not been recognized. Moraines are composed of poorly sorted, but stratified, sediment and few tills. Diamicton units show evidence of water reworking. This research documents a detailed record of MIS 2 glaciation in a maritime setting in western North America and provides evidence of rapid MIS 2 ice-marginal fluctuations that likely reflect responses to millennial-scale climatic fluctuations and may be relevant to understanding other complex MIS 2 moraine sequences.

Climate-driven ecological stability as a globally shared cause of Late Quaternary megafaunal extinctions: the Plaids and Stripes Hypothesis.

ABSTRACTControversy persists about why so many large‐bodied mammal species went extinct around the end of the last ice age. Resolving this is important for understanding extinction processes in general, for assessing the ecological roles of humans, and for conserving remaining megafaunal species, many of which are endangered today. Here we explore an integrative hypothesis that asserts that an underlying cause of Late Quaternary megafaunal extinctions was a fundamental shift in the spatio‐temporal fabric of ecosystems worldwide. This shift was triggered by the loss of the millennial‐scale climate fluctuations that were characteristic of the ice age but ceased approximately 11700 years ago on most continents. Under ice‐age conditions, which prevailed for much of the preceding 2.6 Ma, these radical and rapid climate changes prevented many ecosystems from fully equilibrating with their contemporary climates. Instead of today's ‘striped’ world in which species' ranges have equilibrated with gradients of temperature, moisture, and seasonality, the ice‐age world was a disequilibrial ‘plaid’ in which species' ranges shifted rapidly and repeatedly over time and space, rarely catching up with contemporary climate. In the transient ecosystems that resulted, certain physiological, anatomical, and ecological attributes shared by megafaunal species pre‐adapted them for success. These traits included greater metabolic and locomotory efficiency, increased resistance to starvation, longer life spans, greater sensory ranges, and the ability to be nomadic or migratory. When the plaid world of the ice age ended, many of the advantages of being large were either lost or became disadvantages. For instance in a striped world, the low population densities and slow reproductive rates associated with large body size reduced the resiliency of megafaunal species to population bottlenecks. As the ice age ended, the downsides of being large in striped environments lowered the extinction thresholds of megafauna worldwide, which then increased the vulnerability of individual species to a variety of proximate threats they had previously tolerated, such as human predation, competition with other species, and habitat loss. For many megafaunal species, the plaid‐to‐stripes transition may have been near the base of a hierarchy of extinction causes whose relative importances varied geographically, temporally, and taxonomically.

Orbital- and suborbital-scale changes in the East Asian summer monsoon since the last deglaciation

The variability of the East Asian summer monsoon (EASM) has far-reaching effects on the global climate system and the environment, and full understanding of the variability and dynamics of the EASM contributes to predictions of its future behavior. Here, we present a well-dated mineralogical and total organic carbon record from a saline inland lake in northern China which provides a robust archive of the EASM evolution since 16.0 cal. ka BP. Our record reveals a series of rapid and frequent millennial-scale climatic fluctuations during the last deglaciation; these fluctuations are documented by changes in the abundances of mirabilite, bloedite, and gypsum, which appear to record the Oldest Dryas, the Bølling-Allerød warm period, and the Younger Dryas. The peak EASM moisture occurred in the early and middle Holocene, which was punctuated by a prominent and abrupt weak monsoon interval that occurred synchronously with the 8.2 cal. ka BP cold event. This moisture maximum was terminated at 6.9–5.9 cal. ka BP by a warm-dry event marked by the deposition of gaylussite. Subsequently, the EASM gradually weakened over the late Holocene. The EASM moisture patterns reconstructed from Anguli-nuur Lake display good consistency with records from northern China, as revealed by a regional comparison; moreover, the recorded changes are synchronous with those of the Indian summer monsoon moisture patterns, as revealed by a comparison with the stalagmite records of southern China. Our reconstruction shows that the EASM has responded broadly to Northern Hemisphere summer insolation forcing on orbital time scales since the last deglaciation; thus, insolation is the primary factor that controls regional hydrological variations in the Asian monsoonal domain. The suborbital-scale events are related to the Atlantic meridional overturning circulation, and a slowdown of this circulation would lead to a southward shift of the intertropical convergence zone and a weakening of the EASM.

Deglaciation of the North Cascade Range, Washington and British Columbia, from the Last Glacial Maximum to the Holocene

Glacial retreat from the North Cascade Range after the Last Glacial Maximum (LGM) at approximately 21 ka until the end of the Pleistocene at 11.6 ka was complex and included both continental and alpine glaciers. Alpine valley glaciers reached their maximum extent before 21.4 ka, then underwent a punctuated retreat to valley heads. In the south, beyond the reach of ice sheet glaciation, several end moraines were deposited after the LGM. Moraines marking a re-advance of alpine glaciers to <5 km below modern glaciers were deposited from 13.7 to 11.6 ka.The Cordilleran Ice Sheet flowed south from near 52° north latitude in British Columbia into the North Cascades. At its maximum size the ice sheet covered more than 500 km2 and had a surface elevation of 2200 m in upper Skagit valley. Deglaciation commenced about 16 ka by frontal retreat of ice flanking the mountains. Surface lowering eventually exposed regional hydrologic divides and stranded ice masses more than 1000 m thick in valleys. Isolated fragments of the ice sheet disintegrated rapidly from 14.5 to 13.5 ka, with the pattern of deglaciation in each valley controlled by valley orientation, topography, and climate. Like alpine glaciers to the south, retreat of the ice sheet remnants was slowed by millennial scale climate fluctuations that produced at least one large recessional moraine, and multiple lateral moraines and kame terraces from elevations of 200-1400 m in most valleys. Large volumes of glacial meltwater flowed through the North Cascades and was concentrated in the Skagit and Methow rivers. Outburst floods from deep proglacial lakes spilled across divides and down steep canyons, depositing coarse gravel terraces and alluvial fans at valley junctions.Climate at the LGM was characterized by a mean summer temperature 6 to 7 ºC cooler than today, and 40% lower mean annual precipitation. Persistence of this climate for thousands of years before the LGM caused a 750-1000 m decrease in alpine glacier equilibrium line altitudes (ELA). In the southern North Cascades at 16 ka, glacial ELAs were 500-700 m lower than today, and during advances from 13.7 to 11.6 ka alpine glacier ELAs were 200-400 m lower.

Glacial-to-Holocene evolution of sea surface temperature and surface circulation in the subarctic northwest Pacific and the Western Bering Sea

It has been proposed that North Pacific sea surface temperature (SST) evolution was intimately linked to North Atlantic climate oscillations during the last glacial-interglacial transition. However, during the early deglaciation and the Last Glacial Maximum, the SST development in the subarctic northwest Pacific and the Bering Sea is poorly constrained as most existing deglacial SST records are based on alkenone paleothermometry, which is limited prior to 15 ka B.P. in the subarctic North Pacific realm. By applying the TEXL86 temperature proxy we obtain glacial-Holocene-SST records for the marginal northwest Pacific and the Western Bering Sea. Our TEXL86-based records and existing alkenone data suggest that during the past 15.5 ka, SSTs in the northwest Pacific and the Western Bering Sea closely followed millennial-scale climate fluctuations known from Greenland ice cores, indicating rapid atmospheric teleconnections with abrupt climate changes in the North Atlantic. Our SST reconstructions indicate that in the Western Bering Sea SSTs drop significantly during Heinrich Stadial 1 (HS1), similar to the known North Atlantic climate history. In contrast, progressively rising SST in the northwest Pacific is different to the North Atlantic climate development during HS1. Similarities between the northwest Pacific SST and climate records from the Gulf of Alaska point to a stronger influence of Alaskan Stream waters connecting the eastern and western basin of the North Pacific during this time. During the Holocene, dissimilar climate trends point to reduced influence of the Alaskan Stream in the northwest Pacific.

Reviewing the Lateglacial–Holocene transition in NW Iberia: A palaeoecological approach based on the comparison between dissimilar regions

Isotopic records from Greenland ice cores have revealed a number of centennial to millennial-scale climatic fluctuations affecting the eastern North Atlantic during the Lateglacial/Holocene transition. Most of them appear directly correlated with quantitative temperature changes reconstructed for a very climate-sensitive locality in NW Iberia, Laguna de La Roya (LR). The chironomid-inferred LR temperature record was used as a reference to update the chronologies and improve the interpretations of 16 different key pollen sites from NW Iberia. Botanical and zoological macroremains from archaeological sites and natural deposits were also reviewed in order to check if they support the changing environments predicted by pollen data. Pollen evidence corresponding to the less intense Lateglacial and Early Holocene cold phases identified in LR was found at all sites. Nevertheless, only minor vegetation changes occurred during the Older Dryas, La Roya-I (13,600–13,350 cal BP), La Roya-II (13,300–12,900 cal BP), the 11.4 ka and the 9.3 ka event at very sensitive sites located close to ecotonal situations. We conclude that the LR chironomid inferred-temperature curve is able to explain the main tree-line oscillations and also the major local/regional vegetation changes that occurred over the whole of NW Iberia during the Lateglacial and Early Holocene. During this period, the most relevant landscape modifications occurred at the onset of the Lateglacial, at the transition to the YD, and at the onset of the Holocene. However, noticeable changes also occurred during the 8.2 ka event, which mostly affected the hyperoceanic coastal mountain ranges and some continental depressions inland. The rapid succession of changes registered during the Early Holocene affected the different co-existing biocoenoses in an unequal way and resulted in a diversity of plant and animal resources available for human populations. This may explain that the hunter–gatherer groups even persisted in some areas of NW Iberia until the Mid Holocene.

Climate fluctuation on a kiloyear scale during the Late Last Glacial in Mu Us Desert, China: evidence from Rb and Sr contents and ratios

The MGS2 segment of the Milanggouwan stratigraphical section in China’s Salawusu River Valley records 5.5 sedimentary cycles consisting of dune sands alternating with fluviolacustrine facies or/and palaeosols. The high Rb and Sr contents and low Rb/Sr ratios in the fluviolacustrine facies indicate the presence of a warm and humid climate, and vice versa for a dry and cold climate. Rb and Sr appeared to have 5.5 element cycles that are consistent with the sedimentation changes, and each cycle lasts about 2 ka on average. This study suggests that the observed cycles mainly resulted from variations in the strength of the East Asian winter and summer monsoons, and the MGS2 segment experienced six cold-dry winter monsoons and five warm-humid summer monsoons during the OIS2. In addition, the millennial-scale monsoonal climate fluctuations revealed by the element cycles corresponded well with the Dansgaard–Oeschger cycles recorded in the Greenland ice cores and Heinrich events in the North Atlantic marine sediments. Therefore, the monsoonal climate fluctuations revealed by the Rb and Sr in the MGS2 segment were likely triggered by global climate change.

Iain Stewart Receives 2013 Athelstan Spilhaus Award: Response

It is really wonderful to receive this award and the recognition of AGU for my work. I've been a member of the Union since my Ph.D. days, 25 years or so ago, when I was working on earthquake faulting in Greece and Turkey. Its meetings were always the place that showcased geoscience at its most novel and exciting. It was the breadth of what was on offer that was so exhilarating. My doctoral studies had already opened up fairly wide vistas for me since my interest in Holocene earthquake activity also required an understanding of the Mediterranean's rich cultural history and the vagaries of millennial‐scale climate fluctuations. Later, my research would distract me into farther‐flung realms, such as volcanic faulting on Mount Etna, paleotsunamis along Aegean shores, even postglacial earthquakes in northern Scotland! Kindly colleagues would occasionally advise me to focus my efforts on a single substantive area of research, cautioning that my flighty research forays weren't conducive to climbing the academic ladder. I'm sure they were correct. Or would have been proved so if I hadn't been rescued by television.

Rapid climatic changes and resilient vegetation during the Lateglacial and Holocene in a continental region of south-western Europe

Palynological, sedimentological and geochemical analyses performed on the Villarquemado paleolake sequence (987m a.s.l, 40°30′N; 1°18′W) reveal the vegetation dynamics and climate variability in continental Iberia over the last 13,500calyrBP. The Lateglacial and early Holocene periods are characterized by arid conditions with a stable landscape dominated by pinewoods and steppe until ca. 7780calyrBP, despite sedimentological evidence for large paleohydrological fluctuations in the paleolake. The most humid phase occurred between ca. 7780 and 5000calyrBP and was characterized by the maximum spread of mesophytes (e.g., Betula, Corylus, Quercus faginea type), the expansion of a mixed Mediterranean oak woodland with evergreen Quercus as dominant forest communities and more frequent higher lake level periods. The return of a dense pinewood synchronous with the depletion of mesophytes characterizes the mid-late Holocene transition (ca. 5000calyrBP) most likely as a consequence of an increasing aridity that coincides with the reappearance of a shallow, carbonate wetland environment. The paleohydrological and vegetation evolution shows similarities with other continental Mediterranean areas of Iberia and demonstrates a marked resilience of terrestrial vegetation and gradual responses to millennial-scale climate fluctuations. Human impact is negligible until the Ibero-Roman period (ca. 2500calyrBP) when a major deforestation occurred in the nearby pine forest. The last 1500years are characterized by increasing landscape management, mainly associated with grazing practices shaping the current landscape.

Millennial-scale climate variability during the mid-Pleistocene transition period in the northern South China Sea

Stable isotopes of foraminiferal multispecies (surface dwelling Globigerinoides ruber, thermocline dwelling Pulleniatina obliquiloculata, and benthic Uvigerina peregrina) and Mg/Ca ratios of G. ruber shells were analyzed with an average time resolution of ∼370 yr for reconstructing the orbital- and millennial-scale changes in the East Asian monsoon and associated upper water structure during the mid-Pleistocene period of 800–1060 ka at ODP Site 1144 in the northern South China Sea (SCS). It has been found that generally the sea surface temperature (SST) were lower and the depth of thermocline (DOT) was deeper during glacial stages and vice versa during interglacial stages, indicating a strengthened winter monsoon and weakened summer monsoon during glacial stages. The sea surface salinity (SSS) was relatively lower during glacials, induced by the greatly reduced distance of this site to the shore during times of low sea level. Further, spectral analyses have revealed significant semi-precessional and/or precessional cycles in the planktic δ18O, SST and the proxies of SSS and DOT, showing the typical characteristics of tropical climate change. This means that during the mid-Pleistocene climate transition (MPT) period the East Asian monsoon evolution and associated changes of upper water column structure in the northern SCS were probably driven partly by tropical forcing, like in the southern SCS, and should not be related only to the high latitude ice sheet changes. Particularly, millennial-scale climate fluctuations (mainly ∼1.4 kyr cycle) were found to exist throughout the glacial and interglacial stages during the MPT period in the northern SCS, but slightly different from the sawtooth-like D/O events recorded in late Quaternary Greenland ice cores. During the mid-Pleistocene terminations, the SST warming was synchronous with the northern ice sheet retreat, indicated by benthic δ18O, in the northern SCS, which is consistent to the previous findings in the late Quaternary SCS and apparently different from that in the western equatorial Pacific Ocean, providing new insights into the studies of East Asian monsoon evolution and rapid climate change.

Masked millennial-scale climate fluctuations in SW African during the last glacial

Masked millennial-scale climate fluctuations in SW African during the last glacial

High-Latitude Forcing of the South American Summer Monsoon During the Last Glacial

The climate of the Last Glacial period (10,000 to 110,000 years ago) was characterized by rapid millennial-scale climate fluctuations termed Dansgaard/Oeschger (D/O) and Heinrich events. We present results from a speleothem-derived proxy of the South American summer monsoon (SASM) from 16,000 to 50,000 years ago that demonstrate the occurrence of D/O cycles and Heinrich events. This tropical Southern Hemisphere monsoon reconstruction illustrates an antiphase relationship to Northern Hemisphere monsoon intensity at the millennial scale. Our results also show an influence of Antarctic millennial-scale climate fluctuations on the SASM. This high-resolution, precisely dated, tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the Northern and Southern Hemispheres.

Hydrographic changes in the eastern subpolar North Atlantic during the last deglaciation

Millennial-scale climate fluctuations of the last deglaciation have been tied to abrupt changes in the Atlantic Meridional Overturning Circulation (MOC). A key to understanding mechanisms of MOC collapse and recovery is the documentation of upper ocean hydrographic changes in the vicinity of North Atlantic deep convection sites. Here we present new high-resolution ocean temperature and δ 18O sw records spanning the last deglaciation from an eastern subpolar North Atlantic site that lies along the flow path of the North Atlantic Current, approaching deep convection sites in the Labrador and Greenland-Iceland-Norwegian (GIN) Seas. High-resolution temperature and δ 18O sw records from subpolar Site 980 help track the movement of the subpolar/subtropical front associated with temperature and Atlantic MOC changes throughout the last deglaciation. Distinct δ 18O sw minima during Heinrich 1 (H1) and the Younger Dryas (YD) correspond with peaks in ice-rafted debris and periods of reduced Atlantic MOC, indicating the presence of melt water in this region that could have contributed to MOC reductions during these intervals. Increased tropical and subtropical δ 18O sw during these periods of apparent freshening in the subpolar North Atlantic suggest a buildup of salt at low latitudes that served as a negative feedback on reduced Atlantic MOC.

Cosmogenic 10Be and 26Al ages of Holocene moraines in southern Norway II: evidence for individualistic responses of high-altitude glaciers to millennial-scale climatic fluctuations

Abstract:10Be and 26Al exposure ages are reported for boulder moraine ridges in front of two high-altitude cirque glaciers (Austanbotnbreen, Jotunheimen and Østre Tundradalskyrkjabre, Breheimen) in southern Norway. Ages and 1σ external uncertainties for Austanbotnbreen moraines range from 7.4 ± 0.75 ka to 8.7 ± 0.9 ka when no adjustment for snow shielding is made. With the maximum conceivable snow cover effect (2 m thick, a density of 0.2 g/cm3 , 6 months duration) at the exposed site, the resulting adjusted (7.7 ± 0.8 to 10.9 ± 1.3 ka) as well as unadjusted ages suggest moraine formation possibly during both the 8.2 ka event (Finse Event in southern Norway) and the Erdalen Event ( c. 10.0 ka). In contrast, ages obtained for the outer ridges of the large moraine complex in front of Østre Tundradalskyrkjabre range from 1.1 ± 0.3 ka to 1.9 ± 0.4 ka assuming no snow shielding. Assuming extreme snow-lie conditions (4 m thick, 0.2 g/cm3 density, 6 months duration) at this more sheltered site, the adjusted ages (1.4 ± 0.4 to 2.4 ± 0.5 ka) still indicate late-Holocene, pre-`Little Ice Age' ridge formation. Less likely alternative explanations of the late-Holocene ages, including avalanching of debris onto the moraine, inventories of inherited nuclides in the samples and possible episodic `push-deformation' disturbance of the ice-cored moraine complex, are discussed. The results point to the potential of surface exposure dating in reconstructing Holocene glacier variation chronologies, particularly with respect to high-altitude glaciers and to individualistic responses of these glaciers to Holocene millennial-scale climatic fluctuations. Difficulties with dating moraines in front of small high-altitude glaciers using cosmogenic nuclides, including uncertain snow shielding effect, short glacial transport distance, possibly not `zeroing' an inherited cosmogenic signal and repeated disturbance of a moraine complex, are discussed.