18O Record Research Articles

Last Glacial to Holocene palaeoenvironmental evolution at Lago di Pergusa (Sicily, Southern Italy) as inferred by pollen, microcharcoal, and stable isotopes

Sediments of Last Glacial and Holocene age from two different locations in the basin of Lago di Pergusa have been palynologically and isotopically investigated. New data have been integrated with those available from previous researches carried out by the authors. This study indicates once more that Lago di Pergusa was and still is very sensitive to climate changes. In particular, during the Last Glacial, it played an important role in preserving the biodiversity as thermophilous and mesophilous taxa never completely disappeared. In particular, Zelkova pollen indicates that the distribution area of this tree (still living as a relic plant in south-eastern Sicily) was wider during the Last Glacial, where its last spread is found around 20,000 years BP. The δ 18O record, mirroring the arboreal concentration trend, confirms the climate inferences based on vegetation features. Both the Late Glacial and the Postglacial reforestation periods show slower vegetation dynamics and forest development than expected, with trends mirroring the slow and sustained decline of δ 18O record. The Younger Dryas oscillation is not pronounced in any of the lacustrine palaeoenvironmental proxies, its termination occurring around 11,100 cal. years BP. The opening of the woodland recorded by a slow trend ranging from the Neolithic (around 8000 cal. years BP) to the Eneolithic (few centuries before 4000 cal. years BP) seems to be a natural forest clearance and not due to human impact. Anomalies in pollen, microcharcoals and positive correlations between δ 18O and AP curves are found after a climate change occurring at 4500 years BP, and interpreted as a signal of human impact. The new δ 18O data presented in this paper throws new light to propose this hypothesis, confirming an early aridification trend detected from pollen, and a relatively late human impact inferred from fire dynamics in this central and montane region of Sicily.

Antarctic link to deep flow speed variation during Marine Isotope Stage 3 in the western North Atlantic

The Western Boundary Undercurrent (WBUC), off eastern America, is an important component of the Atlantic Meridional Overturning circulation and is the principal route for southward transport of North Atlantic waters and southward return of Southern Source Water (SSW). Here a direct flow speed proxy (mean grain size of the sortable silt) is used to infer the vigour of flow of the palaeo-WBUC at Blake Outer Ridge, (ODP Site 1060, depth 3481 m) during Marine Isotope Stage (MIS) 3. The overall shape of the flow speed proxy record shows a complex pattern of variability, with generally more vigorous flow and larger-scale flow variations between 35 and 60 ka than in the younger part of MIS 3 and MIS 2 (< 35 ka). Six events of reduced bottom flow vigour (Slow Events, SEs) occur. These appear uncorrelated with Heinrich events, but are instead synchronous with the warming phases of Antarctic Warm Events A-1 to A-4 (with one new one, A-1a and one poorly defined, ‘A-0’). This indicates that Antarctic climate exerts a stronger control on deep flow vigour in the North Atlantic during MIS 3 than Northern Hemisphere climate. The correspondence of SEs with Antarctic warming suggests a weaker WBUC flow due to reduced volume flux at SSW source or reduced SSW density. Because the variability of the lower limb of the WBUC was not connected to sharp North Atlantic changes in temperature, it is unlikely that the Dansgaard/Oeschger cycles were associated with a mode of MOC variation involving whole-ocean overturn, but more likely with perturbations of only the shallow Glacial Gulf Stream–Glacial Northern Source Intermediate Water cell. Nutrient proxies (benthic carbon isotopes and Cd/Ca of Uvigerina peregrina) at this site show similar trends to the GRIP δ 18O record. This correlation has previously been attributed mainly to hydrographic and flow changes but is here shown to be better explained by variations in surface ocean productivity and subsequent decomposition of 12C rich organic material on the sea floor.

Interdecadal climate variability in the Coral Sea since 1708 A.D.

Low resolution (5-year) Sr/Ca and δ 18O samples, extending back to 1708 A.D., were analysed from a Porites coral core collected from Flinders Reef, an offshore reef on the Queensland Plateau in the western Coral Sea (17.5° S, 148.3° E). Using the Sr/Ca ratio as a proxy for sea surface temperature (SST), we deconvolved a salinity record by subtracting the SST signal from the δ 18O record. Decadal variability in the reconstructed salinity record is closely paralleled by changes in SST, with cooler (warmer) temperatures recorded during wetter (drier) periods. This relationship differs from the conventional view often described for tropical areas, where warm temperatures are associated with wet periods and cool temperatures with dry periods. The anti-correlation between reconstructed SST and salinity observed at Flinders Reef, however, matches the climatic effects expected from variations in the Interdecadal Pacific Oscillation (IPO), a recurrent pattern of SST variability over the Pacific Ocean which is known to modulate Australia's climate, in particular the impact of ENSO events on decadal time scales. On longer timescales, salinity seems to have remained almost constant for the last two centuries after a progressive freshening of surface waters that culminated around 1800 A.D. Conversely, SSTs show a warming trend towards the late 20th century.

Contourites of the Gulf of Cadiz: A high-resolution record of the paleocirculation of the Mediterranean outflow water during the last 50,000 years

The Mediterranean outflow water (MOW) paleocirculation during the last 50,000 years has been inferred from the grain-size distribution of contourite beds in core MD99-2341 from the Gulf of Cadiz (Southern Iberian Margin–Atlantic Ocean). Three main contourite facies are described. Their vertical succession defines two contourite sequences that reveal past variations of the MOW bottom-current velocity. A comparison of contourite sequences and the planktonic δ 18O record of core MD99-2341 with the δ 18O record from Greenland Ice Core GISP2 show a close correlation of sea-surface water conditions and deep-sea contouritic sedimentation in the Gulf of Cadiz with Northern Hemisphere climate variability on millennial timescales. A high MOW velocity prevailed during Dansgaard-Oeschger stadials, Heinrich events and the Younger Dryas cold climatic interval. The MOW velocity was comparatively low during the warm Dansgaard-Oeschger interstadials, Bølling-Allerød and the Early Holocene. Rapid sea-level fluctuations on the order of 35 m during Marine Oxygen Isotope Stage 3 are considered to have exerted limiting controls on the MOW volume transport and thus positively modulated the MOW behaviour during the last 50 kyr.

Southern Ocean control on the extent of denitrification in the southeast Pacific over the last 70 ka

Temporal changes in oceanic denitrification, the bacterial reduction of nitrate under suboxic conditions, highlight the potential importance of N inventory changes and the production of N 2O on the climate system. At the same time, the cause of the globally observed variation in denitrification remains unclear. High-resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from ODP Site 1234 on the Chile Margin record integrated denitrification changes within the Peru–Chile Upwelling system over the last ∼70 ka. Denitrification changes in the southeast Pacific are coherent with Antarctic climate changes recorded by the Byrd ice core δ 18O record, and lead northern hemisphere climate events. The southern-hemisphere character of the Chile margin δ 15N record suggests that episodes of reduced denitrification in the SE Pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of Subantarctic Mode Water (SAMW), which forms in the Subantarctic zone of the Southern Ocean and feeds into the low-latitude thermocline.

Glaciation, erosion, and landscape evolution of Iceland

Stratigraphic and sedimentological studies indicate that Iceland has experienced over 20 glaciations during the last 4–5 Myr, in reasonable agreement with the number of glaciations reconstructed from the ∂18O record in deep-sea sediment. The pattern of glacial erosion was to a large part controlled by constructive volcanic processes resulting in increased topographic relief after 2.5 Myr. Between 2.5 and 0.5 Ma valleys up to 400 m deep were excavated into the Tertiary basalts of eastern and south Iceland with an average erosion rate of 10–20 cm ka−1. During the last 0.5 million years rates of erosion increased to 50–175 cm ka−1, with an additional 200 to over 1000 m of valley excavation. Previous estimates of the rate of landscape erosion during the Holocene vary widely, from 5 to 70,000 cm ka−1. We present new studies that define the rates of landscape denudation during the major part of the Holocene (the last 10,200 years): one based on the Iceland shelf sediment record, the other from the sediment record in the glacier-fed lake, Hvítárvatn. Both studies indicate average Holocene erosion rates of about 5 cm ka−1 similar to our erosion rate estimate for 4–5 Ma old strata that has not been subjected to regional glaciation.

The oxygen isotope composition of planktonic foraminifera from the Cariaco Basin, Venezuela: Seasonal and interannual variations

Material collected during a three-year sediment trapping experiment in the Cariaco Basin, Venezuela (January 1997 to December 1999) is used to examine both temporal and inter-species variability in the oxygen isotope composition of planktonic foraminifera. Specifically, this study compares the oxygen isotope composition of six species of planktonic foraminifera ( Globigerinoides ruber (pink), Globigerina bulloides, Neogloboquadrina dutertrei, Orbulina universa, Globorotalia menardii and Globorotalia crassaformis) with the climatology and hydrography of the region, and evaluates the application of each species for use in paleoceanographic reconstructions. The isotope results are consistent with known depth habitats for all six species. The lowest δ 18O values (− 1 to − 2‰) were measured on G. ruber (pink) and G. bulloides, two species that live in the surface mixed layer. Values for deeper-dwelling species such as N. dutertrei, G. menardii and G. crassaformis are higher, predominantly ranging from 0 to − 0.5‰. Temperature estimates derived using species-specific paleotemperature equations indicate that G. ruber (pink) accurately estimates sea surface temperatures (SSTs) throughout the year, while G. bulloides temperature estimates are similar to measured surface temperatures only during the upwelling season (January–April). For the remainder of the year, the δ 18O-derived temperatures for G. bulloides typically are lower than the measured SST. Although the maximum flux of all species occurs during upwelling, the flux-weighted annual mean isotopic composition of the six species indicates that only G. bulloides is biased towards this season. Therefore, we conclude that the sediment δ 18O record of G. ruber (pink) is most suitable for estimating past values of mean annual SST, while G. bulloides provides information on conditions during spring upwelling. The depth of calcification of N. dutertrei varies seasonally in response to changes in the depths of the thermocline and chlorophyll maximum. As a result, the δ 18O difference between G. ruber (pink) and N. dutertrei provides an estimate of the annual surface to thermocline temperature gradient in the basin.

Carbon, oxygen and strontium isotope records of Devonian brachiopod shell calcite

Devonian brachiopod shells from North America, Spain, Morocco, Siberia, China and Germany were analysed for δ 13C, δ 18O and 87Sr/ 86Sr. The samples were screened for diagenetic alteration by means of cathodoluminescence, trace element chemistry (Sr, Mn, Fe) and scanning electron microscopy. 409 out of 1678 shells ranging from the middle Emsian to the middle Famennian were accepted as well-preserved. 87Sr/ 86Sr ratios start with values of 0.70860 in the Lochkovian and decline continuously to 0.70782 at the Emsian–Eifelian boundary. This decrease is attributed to a reduced riverine strontium flux caused by the termination of the Caledonian orogeny. For the Eifelian and Givetian, relatively uniform values between 0.70782 and 0.70784 are observed. 87Sr/ 86Sr ratios start to rise again in the late Givetian and reach a value of 0.70807 in the early Famennian. An enhanced continental delivery of strontium as consequence of tectonic uplift due to the Eovariscan orogeny, in combination with climatic warming, as suggested by the δ 18O record, is interpreted to be responsible for the shift to more radiogenic strontium isotope values. The carbon isotope curve shows four positive δ 13C excursions with amplitudes ranging from + 2.0‰ and + 3.5‰. The positive excursion observed in the australis Zone coincides with the deposition of the Bakoven black shale in New York state. The δ 13C excursion near the Eifelian–Givetian boundary is correlated with the Kacàk event while the two positive excursions in the late Frasnian and at the Frasnian–Fammenian transition are contemporaneous with the deposition of the Kellwasser horizons. All carbon isotope excursions are interpreted to be the result of increased burial of organic carbon. The oxygen isotope record reveals short-term excursions in the late Givetian, late Frasnian and at the Frasnian–Famennian transition which are interpreted to result from changes in surface water temperature. The long-term evolution in δ 18O shows values of − 2.8‰ to − 3.5‰ in the Early and Middle Devonian to generally lower δ 18O values between − 4.3‰ and − 6.1‰ in the latest Givetian and Late Devonian. Assuming a δ 18O of Devonian seawater of − 1‰ V-SMOW, Early and Middle Devonian δ 18O values translate into realistic palaeotemperatures (24 to 27 °C) for tropical to subtropical latitudes. In contrast, the δ 18O values of Late Devonian brachiopods indicate a shift to unrealistically warm palaeotemperatures (31 to 41 °C). The change in δ 18O of brachiopod calcite cannot be explained solely by a change in temperature, pH or by a secular decrease in δ 18O of seawater. A combination of climatic warming and a moderate decrease in δ 18O of seawater from the Middle to Late Devonian may account for the observed decrease in δ 18O of brachiopod calcite. However, this tentative interpretation is not confirmed by conodont apatite δ 18O data that translate into warm but realistic Late Devonian palaeotemperatures.

Devils Hole, Nevada, δ18O record extended to the mid-Holocene

The mid-to-late Pleistocene Devils Hole δ 18O record has been extended from 60,000 to 4500 yr ago. The new δ 18O time series, in conjunction with the one previously published, is shown to be a proxy of Pacific Ocean sea surface temperature (SST) off the coast of California. During marine oxygen isotope stages (MIS) 2 and 6, the Devil Hole and SST time series exhibit a steady warming that began 5000 to > 10,000 yr prior to the last and penultimate deglaciations. Several possible proximate causes for this early warming are evaluated. The magnitude of the peak δ 18O or SST during the last interglacial (LIG) is significantly greater (1 per mill and 2 to 3°C, respectively) than the peak value of these parameters for the Holocene; in contrast, benthic δ 18O records of ice volume show only a few tenths per mill difference in the peak value for these interglacials. Statistical analysis provides an estimate of the large shared information (variation) between the Devils Hole and Eastern Pacific SST time series from ∼ 41 to ∼ 2°N and enforces the concept of a common forcing among all of these records. The extended Devils Hole record adds to evidence of the importance of uplands bordering the eastern Pacific as a source of archives for reconstructing Pacific climate variability.

Rapid fluctuation of alkenone temperature in the southwestern Okhotsk Sea during the past 120 ky

Sea-ice expansion in the Okhotsk Sea in winter is sensitively affected by global warming and cooling. Regionally, the southwestern Okhotsk Sea is closely linked to climate change in East Asia, including Japan, because the cold sea surface temperature (SST) in the southwestern Okhotsk Sea influences directly the development of the Okhotsk atmospheric high-pressure system, and the activated Okhotsk high causes cold climatic conditions in northern Japan. Therefore, environmental change in the Okhotsk Sea indicates two-way interactions as a sensitive mirror reflecting global climate change and as a driving force of regional climate change. To better understand how surface environmental changes in the Okhotsk Sea can influence climate change in East Asia, SSTs were estimated in the southwestern Okhotsk Sea for the past 120 ky with millennial to centennial time resolution using the long-chain unsaturated alkyl ketone (alkenone) thermometer. The alkenone temperature, which corresponds to the SST to 20 m depth in autumn, showed repeated abrupt changes at a centennial timescale, especially during the last glacial period, 20–60 ky before present (BP). The alkenone temperature changed concurrently with changes from interstadials (warm events) to stadials (cold events) in the δ 18O record of the ice cores from Greenland, although some interstadials could not be identified in the alkenone temperature record. A wavelet power spectrum analysis showed that a periodicity of about 8 ky was prominent during 10–90 ky BP, and a 4- to 5-ky cycle was characteristic during 30–40 ky BP in the alkenone temperature records. These periodicities were both similar and dissimilar to those in the Polar Circulation Index, which is based on the atmospheric circulation intensity at high latitudes, as recorded by major-ion concentrations in GISP2. Both the similarity and dissimilarity imply that the SST in the southwestern Okhotsk Sea is controlled mainly by the atmosphere–ocean circulation system in the Northern Hemisphere; however, the relationship between the SST in the Okhotsk Sea and the climate in the Greenland is not linear. Anomalously high alkenone temperatures occurred repeatedly in the glacial period. These warm alkenone temperature episodes would have had multiple causes. In particular, high alkenone temperatures during the last glacial maximum (LGM) have been reported previously for locations near this study site. More investigations are necessary to understand what happened in the Okhotsk Sea and in adjacent seas at the time of the LGM.

Limits to quantifying climate driven changes in denudation rates with cosmogenic radionuclides

The hypothesis that climate change significantly modifies the denudation history of an orogen can be tested by reconstructing catchment-wide denudation histories. Cosmogenic radionuclides measured in sediment deposits of known age (e.g. a sequence of fluvial terraces) offer one approach for quantifying catchment-wide paleo-denudation rates. We present a numerical model, and two case studies, that evaluate the utility of this technique for reconstructing different denudation histories. The concentration of cosmogenic beryllium-10 ( 10Be) is calculated for a sequence of sediments produced from climate driven variations in the mean denudation rate of a drainage basin at sea level and high latitude. The model accounts for the nucleonic and muonic production, and decay of 10Be as sediment is eroded and stored in deposits of different ages. Two scenarios for different climate driven denudation histories are considered: (1) A sinusoidal variation in the input denudation rate with free parameters of periodicity (23, 41, 100 kyr), amplitude (0.1, 0.5, 1.0), and mean denudation rate (0.01, 0.1, 1.0 mm yr − 1 ). (2) A climate driven denudation history based on oscillations of a global benthic foraminiferal oxygen isotope (δ 18O) record over the last 2 Myr. Results from both scenarios suggest cosmogenic radionuclides measured in a system with high denudation rates (> ∼ 0.5 mm yr − 1 ), longer periodicity (100 kyr), and low amplitude have the highest potential for reconstructing climate driven paleo-denudation rates. Unfortunately, the high denudation rates preferred in the theoretical consideration are not suitable for paleo-denudation rate determination due to overprinting of nuclides accumulated during denudation by post-depositional decay and irradiation. Hence, a compromise between the capability to determine changes in paleo-denudation rates and the determination of true denudation rates has to be accepted. Application of the model to previous data sets from northwest Europe and southwest North America produce a good agreement, but highlight the need for future work to consider how geomorphic processes (e.g. landsliding) respond to climate change and influence catchment-wide denudation rates. In addition, other climate proxies than the benthic foraminiferal δ 18O record should be used for specific sample locations.

Soils and palaeo-climate based evidence for irrigation requirements in Norse Greenland

Establishing and sustaining agricultural production was a key factor in the success of Norse settlements during the landnám colonisation across the North Atlantic. In light of the occurrence of channel features in several abandoned home-field areas of the Norse Eastern Settlement of Greenland, and the irrigation requirements of present-day Greenlandic sheep-farmers questions are raised: was irrigation used by the Norse settlers of Greenland on their home-field areas? and, if so, how frequently? Modelling of soil chemical, physical and soil-water hydraulic properties integrated with contemporary high-resolution climatic data demonstrate a frequent requirement for irrigation. Soil moisture deficits are related to the duration and intensity of winter temperature. Using the winter Dye 3 ice core δ 18O record as a climatic proxy, the frequency of moisture deficits, based on comparing mean winter temperatures, indicates that there was a frequent irrigation requirement to maintain home-field productivity, increasing throughout the period of settlement until the 14th Century.

High resolution characterization of the Asian Monsoon between 146,000 and 99,000 years B.P. from Dongge Cave, China and global correlation of events surrounding Termination II

Speleothem samples from Hulu (eastern China, 32°30′N, 119°10′E) and Dongge (southern China, 25°17′N, 108°5′E) Caves provide a nearly continuous record of the Asian monsoon over the last 160 ka [Wang, Y.J., Cheng, H., Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.-C., Dorale, J.A., 2001. A high-resolution absolute-dated Late Pleistocene monsoon record from Hulu Cave, China. Science 294, 2345–2348; Yuan, D., Cheng, H., Edwards, R.L., Dykoski, C.A., Kelly, M.J., Zhang, M., Qing, J., Lin, Y., Wang, Y., Wu, J., Dorale, J.A., An, Z., Cai, Y., 2004. Timing, duration, and transitions of the last interglacial Asian Monsoon. Science 304, 575–578]. We have obtained higher resolution data in the interval between ∼ 99 and 146 ka B.P., providing a detailed account of δ 18O variations over most of MIS 5 and the latter portion of MIS 6. Precise 230Th dating has replicated the chronology of the samples within error. The higher resolution data set confirms the timing of Asian Monsoon Termination II (the midpoint of the negative shift in δ 18O marking the onset of the Last Interglacial Asian Monsoon), placing it at 129.0 ± 0.9 ka B.P. The bulk of this transition (∼ 1.7‰) took place within approximately 70 years, with the total range of the transition being ∼ 3‰. The most abrupt portion of the shift in δ 18O values (∼ 1.1‰) marking the end of the Last Interglacial Asian Monsoon occurred in ∼ 120 years, the midpoint of which is 120.7 ± 1.0 ka B.P. The Dongge Cave monsoon δ 18O record over late MIS 6 exhibits a series of sub-orbital millennial-scale climate shifts that average 1.3‰ in magnitude and occur on average every 1.8 ky. Abrupt shifts in δ 18O of up to 1‰ also occurred throughout the Last Interglacial Asian Monsoon, with periods at multi-decadal to centennial timescales. Similar to the amplitude and periodicities of events found by Dykoski et al. [Dykoski, C.A., Edwards, R.L., Cheng, H., Yuan, D., Cai, Y., Zhang, M., Lin, Y., An, Z., Revenaugh, J., 2005. A high resolution, absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave, China. Earth and Planetary Science Letters 233, 71–86.] during the Holocene in the Dongge record, these shifts cover more than 1/2 of the amplitude of millennial-scale and multi-centennial-scale interstadial events during the Last Glacial Period [Wang, Y.J., Cheng, H., Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.-C., Dorale, J.A., 2001. A high-resolution absolute-dated Late Pleistocene monsoon record from Hulu Cave, China. Science 294, 2345–2348], and millennial-scale and multi-centennial-scale interstadial events during the Penultimate Glacial Period in China (this study). Abrupt decadal to millennial-scale climate events therefore appear to be a general feature of both glacial and interglacial climate. We demonstrate that monsoon intensity correlates well with atmospheric CH 4 concentrations over the transition into the Bølling-Allerød, the Bølling-Allerød, and the Younger Dryas. In addition, we correlate an abrupt jump in CH 4 concentration with Asian Monsoon Termination II. On the basis of this correlation, we conclude that the rise in atmospheric CO 2, Antarctic warming, and the gradual portion of the rise in CH 4 around Termination II occur within our “Weak Monsoon Interval” (WMI), an extended interval of heavy δ 18O between 135.5 ± 1.0 and 129.0 ± 1.0 ka B.P., prior to Asian Monsoon Termination II and Northern Hemisphere warming. Antarctic warming over the millennia immediately preceding abrupt northern warming may result from the “bipolar seesaw” mechanism. As such warming (albeit to a smaller extent) also preceded Asian Monsoon Termination I, the “bipolar seesaw” mechanism may play a critical role in glacial terminations.

Porites corals from Crete (Greece) open a window into Late Miocene (10 Ma) seasonal and interannual climate variability

Variations in the biotic composition of marine shallow water carbonates document global climatic changes. However, a discontinuous stratigraphic record and uncertainties regarding the ages limit the significance of shallow water carbonates as palaeoclimatic archives on geological time-scales. Notwithstanding these deficits, the environmental information stored in the skeleton of reef biota is a unique source of information that resolves seasonal to interannual climate variability in geological time. Application of the method to corals from carbonate rocks is usually restricted to the past 130,000 yr, because the aragonite skeleton undergoes rapid diagenetic alteration. Consequently, reconstructions resolving seasonal to interannual climate variability of the more distant geological time are rare. We describe exceptionally well-preserved corals ( Porites) as old as Late Miocene (10 million years) from the island of Crete (eastern Mediterranean, Greece). Stable isotope records ( δ 18O, δ 13C) reflect seasonal changes in sea surface temperature and symbiont autotrophy. Spectral analysis of a 67-yr coral δ 18O record reveals significant variance at interannual time-scales (2–5 yr) that show similarities to Late Quaternary eastern Mediterranean climate variability. Supported by simulations with a complex atmospheric general circulation model coupled to a mixed-layer ocean model, we suggest that climate dynamics in the eastern Mediterranean may reflect atmospheric variability related to the Icelandic Low 10 million years ago.

Paleoclimate history of Galápagos surface waters over the last 135,000 yr

Records of planktonic and benthic foraminiferal δ 18O and planktonic Mg/Ca from core TR163-22, just northwest of the Galápagos Islands, reveal a detailed (250–450 year resolution) climate history of the region over the last 135 thousand years (kyr). Sea surface temperatures (SSTs), reconstructed from G. ruber Mg/Ca, averaged 24.3±0.4 °C during the Holocene, 22.6±0.6 °C during marine isotope stage (MIS) 2, 3 and 4, and 26.0±0.9 °C during MIS 5e. Changes in SST lead changes in both planktonic and benthic δ 18O by an average ∼3 kyr, suggesting that SST changes in this region predated continental ice volume changes. Changes in SST display clear millennial scale variability, especially in marine isotope stage 3, with behavior somewhat similar to Antarctic proxy air temperature and South Pacific SST records. Removal of the temperature component from the planktonic δ 18O record demonstrates that glacial-interglacial δ 18O-water changes at this site were 1.0±0.2‰, similar to estimates for mean ocean shifts, implying that salinity changes due to regional hydrological variation between the Last Glacial Maximum and Holocene in the Galápagos region were minimal. Comparison between the TR163-22 SST record and an SST record from core TR163-19 North of the Equatorial Front reveals a largely similar broad-scale climate history, suggesting that changes in the Galápagos region were caused by large scale forcing rather than by local or regional dynamical changes. Changes in atmospheric greenhouse forcing are the most plausible explanation for the observed large-scale climate changes in the eastern equatorial Pacific.

A 2200-yr record of hydrologic variability from Foy Lake, Montana, USA, inferred from diatom and geochemical data

A 2200-yr long, high-resolution (∼5 yr) record of drought variability in northwest Montana is inferred from diatoms and δ 18O values of bio-induced carbonate preserved in a varved lacustrine core from Foy Lake. A previously developed model of the diatom response to lake-level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data. High-frequency (decadal) fluctuations in the de-trended δ 18O record mirror variations in wet/dry cycles inferred from Banff tree-rings, demonstrating the sensitivity of the oxygen-isotope values to changes in regional moisture balance. Low frequency (multi-centennial) isotopic changes may be associated with shifts in the seasonal distribution of precipitation. From 200 B.C. to A.D. 800, both diatom and isotope records indicate that climate was dry and lake level low, with poor diatom preservation and high organic carbon: nitrogen ratios. Subsequently, lake level rose slightly, although the climate was drier and more stable than modern conditions. At A.D. 1200, lake level increased to approximately 6 m below present elevation, after which the lake fluctuated between this elevation and full stage, with particularly cool and/or wetter conditions after 1700. The hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake's outlet. Spectral analysis of the δ 18O data indicates that severe droughts occurred with multi-decadal (50 to 70 yr) frequency.

δ 18O record and temperature change over the past 100 years in ice cores on the Tibetan Plateau

The 213 m ice core from the Puruogangri Ice Field on the Tibetan Plateau facilitates the study of the regional temperature changes with its δ 18O record of the past 100 years. Here we combine information from this core with that from the Dasuopu ice core (from the southern Tibetan Plateau), the Guliya ice core (from the northwestern Plateau) and the Dunde ice core (from the north-eastern Plateau) to learn about the regional differences in temperature change across the Tibetan Plateau. The δ 18O changes vary with region on the Plateau, the variations being especially large between South and North and between East and West. Moreover, these four ice cores present increasing δ 18O trends, indicating warming on the Tibetan Plateau over the past 100 years. A comparative study of Northern Hemisphere (NH) temperature changes, the δ 18O-reflected temperature changes on the Plateau, and available meteorological records show consistent trends in overall warming during the past 100 years.

High-resolution ice-volume estimates for the early Miocene: Evidence for a dynamic ice sheet in Antarctica

Ice-volume estimates for the early Miocene (23–16 Ma ATS) were determined by applying δ 18O to sea-level calibrations to high-resolution δ 18O records from ODP Sites 1090 and 1218. These calibrated records indicate that ice-volume ranged between 50% and 125% of the present day East Antarctic Ice Sheet (EAIS) during most of the early Miocene (23–17 Ma). Maximum ice-volume occurred at each of the early Miocene isotopic events (i.e., Mi-events) concomitant with bottom water temperatures generally between ∼1 and 2 °C. Rapid (≪ 1 myr) and high amplitude ice-volume changes also occurred intermittently during this period, with some fluctuations ranging from a fully glaciated East Antarctic continent to a partial collapse of the ice sheet (equivalent to a 50–70% reduction in the EAIS). These large-scale ice-volume changes often occurred at < 100 kyr time scales suggesting an orbitally driven dynamic EAIS existed during the early Miocene. In contrast, the calibrated δ 18O record from Site 1090 indicates significantly less ice-volume (25–70% of the present-day EAIS) was present between 17 and 16 Ma. These results are supported by numerical climate–ice sheet modeling studies that show increased orbitally driven ice-volume variability with elevated levels of atmospheric CO 2. The distribution of bottom water masses during the Miocene was likely significantly different from today, owing to variability in the production of deep-water near the Antarctic continent (i.e., proto-Antarctic Bottom Water, proto-AABW). Weakening of proto-AABW is suggested to have occurred during the early Miocene especially during glacial minima, resulting in it becoming more spatially restricted to the Southern Ocean and becoming entrained into a warmer deep-water mass. Increased production of a warmer bottom water mass is also considered to have been important in the lighter isotopic values observed in the Atlantic and Indian Oceans. These changes are supported by evidence of a large dynamic EAIS based on strata cored on the Antarctic margin, and require a reevaluation of the view that Antarctic ice-volume was significantly reduced for most of the early Miocene. Previous estimates have been constrained by the lack of deep-water isotopic records proximal to Antarctica, and unduly influenced by the expansion of warmer deep-water into most of the ocean basins at this time.

A speleothem record of Younger Dryas cooling, Klamath Mountains, Oregon, USA

A well-dated δ 18O record in a stalagmite from a cave in the Klamath Mountains, Oregon, with a sampling interval of 50 yr, indicates that the climate of this region cooled essentially synchronously with Younger Dryas climate change elsewhere in the Northern Hemisphere. The δ 18O record also indicates significant century-scale temperature variability during the early Holocene. The δ 13C record suggests increasing biomass over the cave through the last deglaciation, with century-scale variability but with little detectable response of vegetation to Younger Dryas cooling.

Eustatic sea-level change at the Mid-Pleistocene climate transition: new evidence from the shallow-marine sediment record of Japan

Abstract Based on the excellent sedimentary record, we have identified five depositional sequences in the upper part of the Omma Formation, exposed along the Japan Sea coast of central Japan. The base of this stratigraphic section is placed just below the top of the Jaramillo Subchron, and the top is located below the Brunhes-Matuyama boundary. Biostratigraphy supports the interpretation that the lower three sequences correlate with marine oxygen isotope stages (MIS) 28–26, 26–24 and 24–22, respectively. We have reconstructed a eustatic sea-level curve from analyses of sedimentary facies, fossil molluscs, basin subsidence and sediment accumulation. The resulting sea-level curve shows that mean eustatic sea-level dropped between 1.0 and 0.9 Ma, and that sea-level was lowest at MIS 22, which has the heaviest δ 18O value between MIS 28 and 22. The total fall in mean sea-level is inferred to be 20–30 m. This independent sea-level record demonstrates that ice-sheet volume rapidly increased at the Mid-Pleistocene Climate Transition (MPT) and culminated in MIS 22, as inferred from the δ 18O record. The upper two depositional sequences imply the contribution of precession to changes in ice sheet volume would have arisen in MIS 21, just after the MPT.