Melanie J. Leng

Freshwater fluxes in the Weddell Gyre: results from δ18O

Full-depth measurements of δ18O from 2008 to 2010 enclosing the Weddell Gyre in the Southern Ocean are used to investigate the regional freshwater budget. Using complementary salinity, nutrients and oxygen data, a four-component mass balance was applied to quantify the relative contributions of meteoric water (precipitation/glacial input), sea-ice melt and saline (oceanic) sources. Combination of freshwater fractions with velocity fields derived from a box inverse analysis enabled the estimation of gyre-scale budgets of both freshwater types, with deep water exports found to dominate the budget. Surface net sea-ice melt and meteoric contributions reach 1.8% and 3.2%, respectively, influenced by the summer sampling period, and −1.7% and +1.7% at depth, indicative of a dominance of sea-ice production over melt and a sizable contribution of shelf waters to deep water mass formation. A net meteoric water export of approximately 37 mSv is determined, commensurate with local estimates of ice sheet outflow and precipitation, and the Weddell 2 Gyre is estimated to be a region of net sea-ice production. These results constitute the first synoptic benchmarking of sea-ice and meteoric exports from the Weddell Gyre, against which future change associated with an accelerating hydrological cycle, ocean climate change and evolving Antarctic glacial mass balance can be determined.

North Pacific seasonality and the glaciation of North America 2.7 million years ago

In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitchs orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago late-summer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.

The tempo of Holocene climatic change in the eastern Mediterranean region: new high-resolution crater-lake sediment data from central Turkey

This study presents results from a multi-proxy analysis of cores taken in a crater-lake sequence from Eski Acigöl in centr-al Turkey which cover the period from pre-c. 16000 cal. yr BP to the present. The sediments comprise an upper unit of enerally non-laminated, banded to massive silts and peats of mid- to late-Holocene age, overlying a laminated unit of late-Pleistocene to early/mid-Holocenie age. The laminae, comprising mainly aragonite. amorphous silica (diatom frustules) and organic matter were formed in a relatively deep, dilute. meromictic lake. Pollen data indicate an abrupt replacement of Arteyisia-chenopod steppe by grass-oak-terebinth parkland during the period of laminae deposition, marking the start of the Holocene. A gradual increase in tree pollen during the early Holocene came to an end c. 6500 cal. yr BP (U-series and adjusted 14C timescale), when mesic deciduots taxa declined at the same time as lake levels fell. Human impact on regional vegetation is inferred from a sharp decline in oak around 4500-4000 cal. yr BP. Diatom, isotopic and mineralogical data indicate that during the second half of the Holocene the lake became relatively shallow and oscillated between fresh and brackish/evaporated water conditions. The contrast between wetter early and drier late-Holocene climatic conditions is matched by other eastern and central Mediterranean proxy climate data. While the Eski Acigöl sequence resembles Holocene hydroclimatic changes in the Saharo-Arabian zone and was also apparenitly linlked to orbital forcing, it is unlikely to have had the samlle direct cause, i.e., an expansion and subsequent retreat of monsoon rainfall.

A dynamic concept for eastern Mediterranean circulation and oxygenation during sapropel formation

We propose that intermittent bottom water ventilation occurred throughout periods of sapropel deposition, and that the recently reported sapropel ‘interruptions’ represent centennial-scale episodes of enhanced frequency/intensity of that process. In essence, the modern high-frequency variability in deep water formation (affected by climatic variability over the northern basins on seasonal to longer time scales) prevailed also at times of sapropel deposition, although the overall ventilation state was much reduced. This concept is supported by: detailed multiple-species isotope records for three Aegean cores; the presence of abundant Globorotalia truncatulinoides within especially sapropels S7 and S8 in the western Levantine basin; observations of three rapid benthic repopulations within sapropel S6 in the deep western Levantine basin; a report of continuous benthic presence through sapropel S1 at intermediate-deep locations offshore Libya; and further supporting information from the literature. In the Aegean records, concomitant abundance of low-oxygen tolerant benthic foraminifera and presence of the more oxyphilic benthic foraminifer Uvigerina mediterranea, with surface-similar δ13C values, indicate repeated deep water re-oxygenation events throughout the deposition of S1. The observations of a continuous benthic presence through S1 (offshore from Libya) imply that no persistent anoxia developed at mid-depth levels in that region, which is far removed from direct deep ventilation influences. The abundance of deep mesopelagic G. truncatulinoides through several sapropels from the western Levantine basin also suggests the presence of bio-available oxygen at many hundreds of meters of depth. Moreover, the rapid/intermittent benthic repopulations within sapropels from the deep eastern Mediterranean imply that bottom water anoxia was spatially restricted and/or of a highly intermittent nature. The short time scales of these repopulation events are incompatible with titration of an extensively anoxic water column and subsequent re-establishment of water-column anoxia. We suggest that where anoxic/azoic conditions were present, they most likely were restricted to a veneer at the sediment/water interface. The extent of such an anoxic ‘blanket’ depends on the balance between advective oxygen supply into the deep sea, and biological and chemical oxygen demand. The demand functions imply a decoupling of oxygenation from water mass advection, allowing export production and Corg posting rates to the sea floor to delimit the extent of the anoxic blanket in both space and time. Low-productivity regions would develop no anoxic blanket, allowing for the observed persistence of deep dwelling planktonic and bottom dwelling benthic faunas.

Mid-Miocene cooling and the extinction of tundra in continental Antarctica

A major obstacle in understanding the evolution of Cenozoic climate has been the lack of well dated terrestrial evidence from high-latitude, glaciated regions. Here, we report the discovery of exceptionally well preserved fossils of lacustrine and terrestrial organisms from the McMurdo Dry Valleys sector of the Transantarctic Mountains for which we have established a precise radiometric chronology. The fossils, which include diatoms, palynomorphs, mosses, ostracodes, and insects, represent the last vestige of a tundra community that inhabited the mountains before stepped cooling that first brought a full polar climate to Antarctica. Paleoecological analyses, 40Ar/39Ar analyses of associated ash fall, and climate inferences from glaciological modeling together suggest that mean summer temperatures in the region cooled by at least 8°C between 14.07 ± 0.05 Ma and 13.85 ± 0.03 Ma. These results provide novel constraints for the timing and amplitude of middle-Miocene cooling in Antarctica and reveal the ecological legacy of this global climate transition.

A high-resolution late Holocene lake isotope record from Turkey and links to North Atlantic and monsoon climate

A high resolution proxy record of precipitation and evaporation variability through the past 1700 yr from δ18O analysis of a varved lake sequence from central Turkey shows rapid shifts between dry periods (AD 300–500 and AD 1400–1950) and wetter intervals (AD 560–750 and AD 1000–1350). Changes are consistent with changes in instrumental and proxy records of the Indian monsoon, dry summers in the Eastern Mediterranean being associated with periods of enhanced monsoon rainfall. In addition major shifts in the record are coherent with changes in North Atlantic winter climate with cold, wet periods in the Alps occurring at times of dry Turkish climate.

Isotopes in palaeoenvironmental research

1. Isotopes in Water W. George Darling, Adrian H. Bath, John J. Gibson, Kazimierz Rozanski Introduction Oxygen and hydrogen stable isotopes in precipitation From precipitation to terrestrial water Lake waters and mass balance modelling Dissolved carbon From proxy to climate - constraints on interpretation Summary References 2. Isotopes in tree rings Danny McCarroll, Neil J. Loader Introduction Tree ring archives Isotope fractionation in trees Sample selection and preparation Mass spectrometry Data analysis Environmental signals Multi-proxy dendroclimatology Summary References 3. Isotopes in bones and teeth Robert E.M. Hedges, Rhiannon E. Stevens, Paul L. Koch Introduction Isotope incorporation into bone Relationship of bone isotope composition to an animals diet Preservation of the isotope signal in bone and tooth Environmental influences on isotope transport through food chains Application of isotope techniques to bone and teeth Summary References 4. Isotopes in lake sediments Melanie J. Leng, Angela L. Lamb, Timothy H.E. Heaton, James D. Marshall,Brent B. Wolfe, Matthew D. Jones, Jonathan A. Holmes, Carol Arrowsmith Introduction Oxygen isotope systematics in inorganic materials Carbon, nitrogen and oxygen in bulk organic sediments and compound specific carbon Nitrogen isotopes in lacustrine organic matter Oxygen and hydrogen isotopes in lacustrine organic matter Summary References 5. Isotopes in speleothems Frank McDermott, Henry Schwarcz, Peter J.Rowe Introduction Oxygen isotopes in speleothems Carbon isotopes in speleothems Fluid inclusions in speleothems: methodologies and some recent results Summary References 6. Isotopes in marine sediments Mark A. Maslin, George E. A. Swann Introduction Oxygen isotopes in marine sediments Carbon isotopes in marine sediments Nitrogen isotopes in marine sediments Silicon isotopes in marine sediments Boron isotopes in marine sediments Summary References Glossary, acronyms, abbreviations Index

Chronology and stratigraphy of Late Quaternary sediments in the Konya basin, Turkey: Results from the KOPAL project

Abstract The Late Quaternary environmental history of the Konya plain, in south central Turkey, is used to examine sediment facies changes in a shallow non-outlet basin which has experienced major climatically driven changes in lake extent. Two principal types of sedimentary archive are used to reconstruct a palaeoenvironmental record, namely alluvial sequences on the Carsamba alluvial fan and sediments from residual lakes. The latter have been used to investigate broader climatic and vegetational histories via palaeolimnological techniques including pollen, diatom and stable isotope analysis. These changes are dated here by radiometric techniques including radiocarbon (AMS and conventional), OSL, and U–Th. Chronological agreement is generally good between the different dating techniques, although typically there is greatly reduced precision beyond ca. 25 ka. Lake sediment cores investigated have basal ages beyond the range of 14C dating, but contain hiatuses as a result of subsequent alternation between phases of lacustrine sedimentation and aeolian deflation. In contrast to most deepwater non-outlet lake systems, the Konya basin may have been occupied by a single extensive lake for as little as 10% of Late Quaternary time, mainly around the time of the LGM. This lake highstand was followed by an important arid interval. In the absence of unbroken chronostratigraphic sequences, palaeohydrological investigation of shallow non-outlet lakes may require analysis of basin-wide changes in sedimentation rather than reliance on single core records. Stratigraphic continuity in such sedimentary environments cannot be assumed, and requires independent chronological control through radiometric dating.

Tracer?derived freshwater composition of the Siberian continental shelf and slope following the extreme Arctic summer of 2007

We investigate the freshwater composition of the shelf and slope of the Arctic Ocean north of the New Siberian Islands using geochemical tracer data (? 18O, Ba, and PO*4) collected following the extreme summer of 2007. We find that the anomalous wind patterns that partly explained the sea ice minimum at this time also led to significant quantities of Pacific?derived surface water in the westernmost part of the Makarov Basin. We also find larger quantities of meteoric water near Lomonosov Ridge than were found in 1995. Dissolved barium is depleted in the upper layers in one region of our study area, probably as a result of biological activity in open waters. Increasingly ice?free conditions compromise the quantitative use of barium as a tracer of river water in the Arctic Ocean.

Early Holocene retreat of the George VI Ice Shelf, Antarctic Peninsula

The recent collapse of several Antarctic Peninsula ice shelves has been linked to rapid regional atmospheric warming during the twentieth century. New high-resolution lake sediment records of Holocene ice-shelf behavior show that the George VI Ice Shelf was absent beginning ca. 9595 calibrated (cal.) yr B.P., but reformed by ca. 7945 cal. yr B.P. This retreat immediately followed a period of maximum Holocene warmth that is recorded in some ice cores and occurred at the same time as an influx of warmer ocean water onto the Antarctic Peninsula shelf. The absence of the ice shelf suggests that early Holocene ocean-atmosphere variability in the Antarctic Peninsula was greater than that measured in recent decades.