PP14B-01
A new Method for Paleotemperature Reconstruction - Noble gas Measurements on Speleothems.
Dissolved atmospheric noble gases in groundwater are used to reconstruct paleotemperatures based on their temperature dependent solubilities. Noble gases dissolved in microscopic water inclusions in speleothems are expected to reflect the cave temperature and thus potentially are a new tool for paleotemperature reconstruction. Noble gases extracted from speleothem samples consist of two components: The noble gases corresponding to the air-equilibrated water and additionally gases from air-filled inclusions. Too large contributions of air can mask the temperature information contained in the water and are a major challenge for the method. Stepwise procedures enable to separate these components partially and thus provide the possibility for temperature determination. Measurements on several speleothems from the Bunker Cave (Germany) enabled the reconstruction of paleotemperatures for three different periods: the Holocene, Marine Isotope Stage 3 and the Eemian interglacial. The results from different speleothems with temporally overlapping growth periods are in reasonable agreement. Multiple measurements on the same growth layer yielded furthermore reproducible noble gas temperatures. The temperature differences obtained from the Bunker Cave stalagmites are coincident with earlier studies based on pollen, sediment and ice cores as well as other speleothem data. Measurement of both the stable isotopes (δ18O, δ13C) and noble gas temperatures on the same stalagmite enable the disentanglement of different effects on the stable isotopes and finally may result in a better understanding of past climate as well as of the isotope signals as climate proxies in speleothems.
PP14B-02
Comparison of Instrumental and GDGT Based Estimates of Sea Surface and Air Temperatures From the Skagerrak: a New Approach to Novel Proxy Validation
One of the greatest challenges in reconstructing past temperatures is to obtain sea surface and air temperatures from the same archive, the ultimate goal being the correlation of continental and ocean temperatures and understanding issues such as the interaction between ocean circulation and land climates. In principle this could be undertaken using glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids derived from Archaea and bacteria occurring ubiquitously in aquatic environments and soils. One of the proxies based on these lipids is the so-called TEX86, calculated by quantifying the average number of cyclopentane rings contained in GDGTs derived from Archaea and increasingly applied to estimate past SST. In 2007 a new tool to reconstruct past annual mean air temperatures (MAT) emerged, which estimates continental air temperature from bacterial lipids in soils (branched GDGTs). This proxy quantifies the degree of cyclisation of the branched GDGTs, expressed as the cyclisation ratio of branched tetraethers (CBT) which correlates with soil pH, and the methylation degree of branched tetraethers (MBT) which depends on soil pH and MAT. Given that GDGT proxies are relatively novel, uncertainties remain on their general applicability and interpretation. In this communication we test the TEX86, MBT and CBT proxies by applying them to a high resolution marine sediment core, and comparing their estimates for the last 200 years with regional instrumental records of sea surface and air temperatures. The average SST estimated from TEX86 in core MD99-2286 for the last 200 years is 11.1°C, values ranging from 9.6 to 13.9°C. This is comparable to the regional annual mean instrumental values, which average 10.0°C and range between 7.5 and 12.2°C. The average MAT estimated from MBT and CBT is 15.6°C, and values range between 13.4 to 16.6°C. These values, rather than with the instrumental annual mean, are in agreement with the instrumental record of regional summer mean air temperature, which averages 15.5°C and ranges between 14.2 and 16.3°C. Both reconstructed sea surface and air temperatures follow the same trend as the instrumental records although there is an apparent offset between them, where the reconstructed records appear to lag the instrumental trend. The density of data points is different in both records, but at this point we think that this probably due to age model uncertainties. These results provide further support for the use of GDGT proxies to estimate past sea surface and air temperatures.
PP14B-03
Reproducibility of a High-Resolution, Late Holocene Foraminiferal Mg/Ca Record From the Gulf of Mexico
The errors associated with Mg/Ca-SST estimates from planktonic foraminifers are critical in the interpretation of low-amplitude decadal to sub-decadal resolution marine records. Inter-test variability within a temporally well-constrained sample can exceed the amplitude of the environmental signal. In this study we examine a number of factors that contribute to the error associated with Mg/Ca records including test size, inter-test variability and intra-test variability. We also test the reproducibility of late Holocene Mg/Ca records both within the same box core from the Pigmy Basin (Gulf of Mexico), and among three different basins in the northern Gulf of Mexico (Garrison Basin, Fisk Basin and Pigmy Basin). Results indicate a significant positive correlation between Mg/Ca and test size, with a range of 1.0 mmol/mol (2.5°C) from the smallest (150- 212μm) to largest (500-600μm) size fractions of Globigerinoides ruber (pink). No significant correlation was observed between test size and Mg/Ca in G. ruber (white), Neogloboquadrina dutertrei or Globorotalia menardii. Within the Pigmy Basin, the downcore Mg/Ca record generated from G. ruber (pink) and G. ruber (white) yielded the same magnitude and pattern of variability over the past 1400 years. Downcore Mg/Ca records generated from G. ruber (white) covering the past 500 years from the 3 Gulf of Mexico basins show a consistent 2.5°C warming from the maximum cooling in the Little Ice Age (c.a.250 yrs BP) to the 20th century.
PP14B-04
Multi-centennial Arctica islandica Master Chronology From the Irish Sea: Radiocarbon Reservoir Corrections, Suess Effect, Stratification Dynamics and Volcanic Eruptions
Detailed understanding, at high spatial and temporal resolutions, of forcings and feedbacks within the climate system have been limited because the temperate marine environment has lacked an absolute timescale of the kind provided by tree-rings for the terrestrial environment and by corals for the tropical marine environment. We present the first annually resolved, multi-centennial (489-year), absolutely dated, shell- based marine master chronology. The chronology has been constructed by detrending and averaging annual growth increment widths in the shells of multiple specimens of the very long-lived bivalve mollusc Arctica islandica, collected from sites to the south and west of the Isle of Man in the Irish Sea. Three different detrending techniques (negative exponential function, segment length dependent spline and regional curve standardization) have been employed to preserve low-frequency signals, and their characteristics compared. The strength of the common environmental signal expressed in the chronology is fully comparable with equivalent statistics for tree-ring chronologies. Analysis of the 14C signal in the shells shows a weakly significant (R2 = 0.13, p = 0.04) decreasing trend in the marine radiocarbon reservoir correction, indicative of a strengthening of the North Atlantic Current or of increasing precipitation (tendency towards positive wNAO phase). The delta 13C signal shows a very significant (R2 = 0.49, p <0.001) trend due to the Suess effect. The chronologies are compared with the 350-year Central England Temperature record and with seawater temperature records from stations close to the study site in the Irish Sea. Significant correlations are found between the chronologies and (a) mean annual air temperatures and summer-autumn SSTs for the previous year (b) late summer-autumn air temperatures and SSTs for the year of growth and (c) the timing of the autumn decline in SST. Changes over time in low-frequency correlations with air temperatures and towards a deeper water origin for the shells in the chronology indicate that shell growth can be used to study seasonal stratification dynamics. Periods of low growth are contemporaneous with major volcanic events at Laki (1783-84) and Tambora (1815-16), suggesting marine ecosystem perturbation linked to inhibited phytoplankton production.
PP14B-05
High resolution climate variability during MIS 5e; SST and oxygen isotope records from the Cariaco Basin, Venezuela
The Cariaco Basin is located on the Northern Venezuelan shelf within the trade wind belt; the seasonal migration of the ITCZ is recorded in modern basin sediments in annually layered varves. Studies have shown the Cariaco Basin to record nearly synchronous changes with the North Atlantic climate on glacial/interglacial timescales. Combined with high sedimentation rates (15 - 30 cm/kyr) and near-permanent anoxia below 250m water depth, this climate sensitivity makes the Cariaco Basin an ideal location for high resolution paleoclimatic studies. By analyzing planktic foraminiferal (G. ruber) calcite in particularly well-preserved sediments during Marine Isotope Stage 5e (MIS 5e; 120 - 129 ky bp) from piston core MD03-2620, we have constructed a multi-decadal resolution record of Mg/Ca sea surface temperature (SST) and d18O. The onset of MIS 5e is characterized by rapid warming; SSTs peak at over 29deg C which we correlate with the CH4 spike in the Vostok ice core at 128.5 ky bp. While SSTs decline to an average of 28degC for a relatively stable interval lasting from 127 - 122 ky bp, the d18O-calcite record is characterized by pronounced millennial scale oscillations of about 0.5 per mil amplitude during this time. These oscillations cannot be temperature driven and therefore must reflect changes in global sea level or local salinity variations driven by ITCZ related rainfall. When compared with Holocene records of climate variability from the Cariaco Basin, our MIS 5e record shows average SSTs approximately 1.5deg C warmer than Holocene values, but with similar magnitude shifts in d18O-calcite. One marked difference between the two records is the structure of the glacial terminations leading into these warm periods; our Termination II record implies the relative timing of climatological events was not constant between these two deglaciations.
PP14B-06
A High Resolution N.E. Atlantic Sediment Record of Termination II and MIS 5e Millennial- scale Variability
The high resolution, long sediment core MD04-2822 was recovered from the Rockall Trough (56°50.54 N, 11°22.96 W; 2344m water depth) in the N.E. Atlantic and is 38m in length. This long sediment core has an expanded MIS 5 sequence and the isotopic shift associated with Termination II is unusually expanded over 2.5m. We explore the interaction of millennial scale variability for this region of the N.E. Atlantic across Termination II and the sub-stage 5e/5d climatic transitions. We present the following proxies: 2mm ITRAX core scanning data, IRD records, planktonic and benthic stable isotopes as well as faunal and Mg/Ca estimates of sea surface temperatures. The stratigraphy for MD04-2822 is securely anchored to the Greenland ice cores by the presence of North Atlantic Ash Zone (NAAZ) I and II; the synchronicity of atmospheric temperature change over Greenland and sea surface temperatures has previously been demonstrated. The core site is currently located within the main axis of the present day warm N. Atlantic current and Mg/Ca sea surface palaeotemperature reconstructions for MIS 5e, based upon the planktonic foraminifera G. bulloides, suggest millennial scale temperature variations of around 2.5°C at this site. The initiation of the NEEM project, to recover MIS 5e age Greenland ice, will necessitate comparison with high resolution marine records, such as we present for MD04-2822, in order to evaluate the stability of an Eemian interglacial climate postulated to be warmer than present. The marine archive therefore provides one valuable means by which the natural climate variability in an interglacial warmer than present may be evaluated.
PP14B-07
The Last Deglaciation Recorded in Central Sierra Nevada Stalagmites, California
The abrupt climatic shifts delineated by the Greenland ice cores are also archived in a precisely dated (U- Th), sub-centennial to sub-decadal resolution speleothem record from a central Sierra Nevada (38° N) cave (Moaning Cave). Repeated shifts in stalagmite stable and radiogenic isotopes and trace elements are coeval with the Younger Dryas and Bölling/Alleröd climatic events and their associated interstadials/stadials. The stalagmite precipitated without visible discontinuity from 18ka to 8ka based on 230Th/U dating. The stable isotope profiles measured along the stalagmite growth axis are reproducible in off-axis transects, consistent with precipitation of the stalagmite in isotopic equilibrium with drip water. The O and C time series display large variations (2‰ for O and 5‰ for C) along the speleothem's growth axis. An abrupt shift to minimum O and C isotope values and Sr, Mg, and Ba concentrations, and less radiogenic, soil-dominated 87Sr/86Sr is coincident, within current dating uncertainties, with the onset of the Younger Dryas (YD) cold event. Conversely, the onset of the Bölling/Alleröd warm period is represented by increases in O and C, elevated Sr, Mg, and Ba concentrations, and more radiogenic, host rock dominated 87Sr/86Sr values. Abrupt shifts to more negative stable isotope values and Sr, Mg, and Ba concentrations, and less radiogenic 87Sr/86Sr occur during the stadials of the Bölling/Alleröd. Lower (higher) O and C isotopes, and Sr, Mg, and Ba concentrations, and less (more) radiogenic 87Sr/86Sr suggest decreased (increased) water-carbonate interactions and prior calcite precipitation and increased (decreased) vegetation density above the cave driven by increased (decreased) precipitation during high-latitude cold (warm) periods. The Moaning Cave record indicates that the effects of the YD event may have been longer lived in the western Sierra Nevada than in Greenland, a result that is consistent with some western North American proxy records that suggest cooler, wetter conditions in Western North America beyond 11,500 years BP. A more complex internal structure of the YD than typically revealed by other proxy records is observed in the Moaning Cave record. The Moaning Cave record provides new insight into the climatic impacts of high- latitude climate shifts in California and interhemispheric teleconnections during abrupt climatic events.
PP14B-08
Monsoonal Rainfall Fluctuations During The Late Quaternary And Its Relation To Marine Isotopic Stages: A Case Study From The Ganga Basin, India
The carbon and oxygen isotope ratios of soil carbonate nodules were measured from 50 m long core drilled out from Kalpi cliff section (Ganga Basin) to reconstruct the monsoonal rainfall and its effect on vegetation. The oxygen isotope ratios of soil carbonate of Kalpi core (Age: 128 to 35 ka) samples ranges from –7.5 to –4.1‰. Maximum lowering of oxygen isotope ratios was observed around 120 and 60 ka. The post 120 ka time period is characterized by continuous enrichment and the highest value was observed at around 70 ka. The depleted values of oxygen isotope ratios around 120 and 60 ka indicate monsoonal rainfall was intensified during this period which is well correlated with Marine Oxygen Isotopic Stages. On the other hand, the ä13C values range from –6.4 to 1.6‰. The ä13C values of soil carbonates suggest that during monsoon intensification vegetation was dominated by C3 type of plants. The one to one correlation between monsoon variations and C3/C4 ratio changes show that atmospheric CO2 which was thought to be one of the controlling factors for appearance and expansion of C4 plants during the late Miocene period did not play a major role in determining the abundances of C3-C4 plants. • Corresponding author: psanyal@gg.iitkgp.ernet.in