PP13A-1416
Reconstructing the timing, flux, and source of Last Glacial loess accumulation in the North American midcontinent
Terrigenous dust can be both a product and an agent of climate change. Ice core records show increased levels of continental dust during glacial periods compared to interglacials, with exceptional levels of dust being recorded during the last glacial maximum (LGM). The terrestrial deposits of wind blown dust known as loess also support these observations, preserving records of changes in climate and in atmospheric circulation, and documenting the changing sources and flux of dust over time. The greatest thicknesses of Last Glacial loess yet identified are preserved in the North American midcontinent, and termed "Peoria Loess". On the basis of thickness alone, these Peoria Loess deposits suggest high dust accumulation rates during the Last Glacial period across a large area of North America. Using optically stimulated luminescence (OSL) dating, we show that mass accumulation rates (MARs) for Peoria Loess in Nebraska and western Iowa are much higher than any other pre-Holocene location worldwide, and that these MARs fluctuate over time. The loess deposits at these sites are derived from both glacigenic and non-glacigenic sources. A combination of optically stimulated luminescence (OSL) dating and geochemistry is employed, providing a powerful tool for the reconstruction of what prove to be dramatic changes in the timing, flux, and source of dust; this information is important for the validation of models evaluating the role of dust in climate change. These loess records may not only serve as passive testimony to the response of dust to climate change, but may also provide evidence to support the active role of dust in forcing climate change.
PP13A-1417
Multidisciplinary Applications of a Regional Radiocarbon age Chronology: a Case Study From the Kenai Peninsula, Alaska
We recovered and examined 25 peat cores from a northeast to southwest transect of the Kenai Peninsula, parallel to the Cook Inlet volcanoes and covering an area of 8,050 km2 (70 km wide by 115 km long). Forty eight AMS radiocarbon ages, ranging from recent to ca. 12,300 yr. B.P., were obtained from Sphagnum sampled directly beneath visible tephra layers. An additional 17 basal AMS radiocarbon ages, ranging from ca. 2,000 yr. B.P. to ca. 19,000 yr. B.P., were dated using organic material at the basal contact. One of these 25 cores is the oldest peat core to date in this region. Ongoing analysis of magnetic susceptibility (MS), field characteristics, stratigraphic context, petrographic classification, geochemical trends (electron microprobe analyses), particle size, shard characteristics, and radiocarbon ages are being used to identify, characterize, and correlate 221 tephra deposits among cores. Important marker tephra layers have been identified, which augment and improve the resolution of the chronology. We are utilizing this tephrostratigraphic framework and chronology to compile a composite tephrostratigraphy, from which to better understand regional tephra-fall frequency and paleo-wind direction, assess regional ash-fall hazards, and link tephra layers to source volcanoes. In addition, these peat cores and our chronology have been utilized by a variety of researchers in varying disciplines studying this region of Alaska. Our chronology has provided these scientists with a framework from which to carry out some critical paleoenvironmental research studies, including carbon dynamics, peat accumulation, glacial geology, and vegetation history. Preliminary findings from these studies will be highlighted to further demonstrate the multidisciplinary applications of a regional radiocarbon age chronology. Together, these researchers are utilizing this chronology to understand the paleoenvironment of Southcentral Alaska, while benefiting from shared resources and knowledge.
PP13A-1418
Vegetation variations and associated environmental changes during the past ~25,000 14C yr BP in the western part of the Chinese Loess Plateau
We present a high-resolution bioclimatic reconstruction from a 40-m thick section in the western part of the Chinese Loess Plateau in attempt to reveal the details of the MIS 2 and MIS 1 climate changes. Our data show that a desert steppe occupied the landscape between ~24,500 and ~13,600 14C yr BP. Two episodes of vegetation deteriorations occurred between ~20,000 and ~17,100 14C yr BP and between ~15,100 and ~13,600 14C yr BP were corresponding to the H2 and H1 events. The deglacial stage between ~13,600 and ~9,880 14C yr BP included four sub-stages that were chronologically corresponding to the European deglacial sub-stages. First sub-stage (~13,600 - ~ 12,400 14C yr BP) was a correspondent of the Bølling warm period, second sub-stage (~12,400 - ~11,500 14C yr BP) to the Older Dryas cold period, third sub- stage (~11,500 - ~10,700 14C yr BP) to the Allerød warm period, and fourth Sub-stage (~10,700 - ~9,880 14C yr BP) to the Younger Dryas cold period. The early and middle Holocene (~9,880 - ~4,370 14C yr BP) was a period when a forest steppe dominated the landscape under a warm and wet climate. During this period, eight cool and dry spells occurred at ~ 9,720 14C yr BP (E1), ~9,380 14C yr BP (E2), ~8,870 14C yr BP (E3), ~8,660 14C yr BP (E4), ~8,440 14C yr BP (E5), ~8,240 - ~8,060 14C yr BP (E6), ~7,120 - ~6,550 14C yr BP (E7), and ~5,640 - ~5,120 14C yr BP (E8). After a drastic deterioration of vegetation between ~4,370 - ~ 3,270 14C yr BP, the study area experienced an improved moisture condition between 3,270 - ~1,450 14C yr BP, followed by a persistent deterioration of vegetation under a drying climate during the past ~1,500 years.
PP13A-1419
Timing for aeolian deposits and paleo high lake levels in the northeastern Tibetan Plateau in China using OSL dating
It has been reported that mega-lakes with much higher lake level existed throughout the Tibetan Plateau and its adjacent areas. In the Qaidam basin lake levels up to c 140 m higher than present have been observed. 14C dating results indicate that there was a high lake level during the Marine Isotope Stage (MIS) 3 in the Qaidam basin, where it is now a flat salt lake playa. It implies that dramatic climate change occurred during or after MIS 3, and that the Qaidam basin soon became an arid place with the formation of halite. The lacustrine deposits contain records of these environmental changes. When the climate shifted from warm-humid to cold-dry conditions, the associated desiccation will result in occurrence of aeolian activity, due to the exposure of vast lacustrine deposits. Aeolian deposits transported by northwest wind started to accumulate in the southeast of the Qaidam basin. As a result, the timing and the spatial distribution of the aeolian sediments in the Qaidam basin will facilitate our understanding of environmental changes. Chronology will play a key role in understanding the evolution of geomorphology and the driving force of the dramatic environmental change. Luminescence dating is using the sediment itself, quartz or feldspars, as dating materials, so to date directly the climate event. Studies of its application to the dating of aeolian and lacustrine deposits in this area has been limited by far. Recently, we undertake a multi-discipline project which aims: (a) to reconstruct the palaeoenvironment in the Qaidam basin; (b) to better understand the timing of aeolian accumulation and salt lake evolution; (c) to understand the timing of the human inhabitation/migration to the Tibetan Plateau. We will report here the up- to-date results of the on-going project, especially the chronological progress from the luminescence dating work.
PP13A-1420
Dating Middle Pleistocene loess using IRSL luminescence
Loess is a unique palaeoclimate proxy that has a relatively global distribution. A major issue in loess studies is their age, as most terrestrial sediments are outside the realm of isotopic dating methods. Luminescence dating of loess has been attempted with limited success as Optically Stimulated Luminescence (OSL) from the two common dosimeters used in luminescence, quartz and feldspar minerals, both yielded age underestimates. Quartz is limited by dose saturation and feldspar suffers from anomalous fading. Over the last decade, we have developed methods to deal with anomalous fading and hence correct Infrared Stimulated Luminescence (IRSL) ages from feldspar dominated samples. A method known as Dose Rate Correction (DRC) has been successfully applied to loess from the Western European Belt, for ages as old as the Middle Pleistocene. Ages using the same method have been obtained for loess in Alaska and the technique is now being extended to loess from Illinois and China. IRSL can also be used as a reliable telecorrelation tool as luminescence properties of loess are broadly similar, whatever the geological provenance. DRC corrected IRSL extends the applicability of luminescence to dating loess up to at least 500 ka. The limiting factor in the specific case of loess is dose saturation due to relatively high dose rate compared to the average terrestrial sediment radioactivity.
PP13A-1421
The last 850 millennia recorded at the Stari Slankamen loess-paleosol sequence: revised chronostratigraphy and long-term environmental trends
The Stari Slankamen loess section is located on the northeastern part of the Srem Loess Plateau (Vojvodina region, North Serbia) on the west bank of the Danube River opposite the Tisa (Tisza) confluence. The ca. 40-m thick cliff is comprised of loess intercalated with 7 major paleo-pedocomplexes and can be considered to be one of the most significant, nearly continuous, Quaternary sections in the Carpathian (Panonnian) Basin area. Here we present magnetostratigraphic and aminostratigraphic evidence that further emphasizes the importance of the site in terms of its age and the long-term paleoclimatic record it preserves. Characteristic remnant magnetization, obtained through alternating field demagnetization, was obtained on 59 oriented samples and demonstrates the presence of the Matuyama-Brunhes boundary (MBB) at a profile depth of 36 m, within loess unit V-L7. This interpretation is confirmed by new high resolution paleomagnetic investigations (434 oriented samples) from the lower of the profile. Low frequency field magnetic susceptibility was measured in situ in the lower 20 m of the exposure and in the laboratory on samples taken from the upper 20 m of the loess-paleosol sequence. As a record of pedogenic alteration, the magnetic susceptibility (MS) record provides a mean of correlating the sequence with key loess sites in Central and Southeastern Europe, China, and with key climate archives such as the marine oxygen-isotope record. The MS records and evidence from amino acid geochronology measurements indicate a missing pedocomlex (V-S2), the result of an erosion event represented by distinct layer overlying an erosion unconformity. In addition, the magnetostratigraphic and aminostratigraphic based age model requires a significant revision of hitherto published chronostratigraphic subdivisions of the sequence. Our revised chronostratigraphic model suggests that previous age estimates, including results of previous thermoluminescence dating, need to be reconsidered, as they underestimate the depositional age of the material. The relative completeness and long time period represented by the Stari Slankamen section is unusual in European loess sequences. As such, the section provides a rare opportunity to investigate detailed and long-term climatic change over the Middle Pleistocene in a region influenced by air masses from high and middle latitudes as well as the North Atlantic and Mediterranean. The changing relative importance of these air masses through time provides insight into local and regional synoptic systems and their evolution through the last ca. 850 ka. The section can thus be considered as one of the key loess - paleoclimate records in Europe.
PP13A-1422
An introduction to aeolian dust dynamics recorded in the Stari Slankamen loess sequence (Vojvodina, Serbia) - Implications for past atmospheric circulation patterns of Eurasia
Loess sequences provide a direct proxy for the sedimentation dynamics of atmospheric dust, giving important information about the origin of the mineral aerosols and past palaeoclimatic conditions at that time of deposition. The conditions that determine the areas of origin, the transport and distribution of the aeolian material, are being accentuated by the dynamic process structures at the interface of the land surface and the atmosphere. Eventually this may allow the reconstruction of past atmospheric circulation patterns through high resolution loess investigations at different loess sites. The Stari Slankamen loess-palaeosol sequence is situated in the eastern part of the Srem Loess Plateau on the right bank of the Danube near by the river mouth of the Tisza into the Danube. Eight loess layers were distinguished, intercalated by nine pedocomplexes. The sequence under study has a total thickness of approximately 45 m. According to previous investigations the Stari Slankamen loess site is considered as one of the most important sections in the Pannonian basin. In this paper we present the first results of highly resolved grain size studies of the loess section Stari Slankamen. During fieldwork samples for grain size were taken at 2 cm intervals from the loess and the palaeosols, respectively. Particle size measurements of all samples were processed on a Beckman Coulter LS 13320 PIDS laser sizer with auto-prep station to provide a dynamic range that spans from 0.04 to 2000 µm and ensure accuracy and reproducibility. The granulometric results show distinct variations within the silt fraction and allow a clear distinction between cold and warm cycles. They show an important coherence between the type of dust sedimentation and the prevailing climate and offer insight into past atmospheric circulations of SE-Europe. Our results fundamentally contribute to a better understanding of interhemispheric teleconnections between Eurasian climate systems.
PP13A-1423
High resolution paleoenvironmental and climate variability of SE Europe since the MIS 9, recorded at the Mošorin loess-paleosol sequence (Vojvodina, Serbia)
Loess-paleosol sequences in the Vojvodina region (Serbia), located in the confluence area of the rivers Danube, Sava and Tisza, are thought to be among the oldest and most complete terrestrial paleoclimatic records in SE Europe. The Mošorin (Veliki surduk exposure) loess record in the north of the Titel loess plateau, represents a unique archive of Pleistocene climate variability, characterized by considerable high sedimentation rates during the last three glacial cycles. This study presents paleopedogenic parameters obtained from samples taken at 5 intervals from the upper 34 m of the Mošorin loess record. For the total number of 732 samples colors were determined. Wet as well as dry colors were observed using Munsell Soil Color Chart. Rubification and melanization indices were calculated following Harden (1982), using spreadsheet developed by Taylor (1988). As Kubiena (1970) define rubification, this soil development index show the degree of soil reddening relative to parent material, and melanization expresses soil darkening. As it was observed at other Euroasian loess deposits, at Mošorin loess-paleosol sequence rubification and melanization values for loess layers meet the expected results. The rubification values are significantly lower then those for paleosols. Melanization indices are much lower for paleosols. Rubification values are increasing with depth and time, while the ones for melanization decrease. This can be explained by increasing mineral weathering, nature of soil formation processes and quality of soil organic mater, although it could be related to geochemical composition of material. The results, conducted from paleopedogenic and geochemical parameters will give a better perspective on paleoclimatic transitions in Vojvodina region during Pleistocene.
PP13A-1424
Glacial-interglacial Climate Variability in the Black Sea Region Since MIS 15: Record of Highly Resolved Particle-size Dynamics From the Loess Sequence Mircea Voda, Romania
The aeolian dust record of the loess sequences in the Dobrogea region, Romania, provides one of the most
complete terrestrial climate records in proximity to the Black Sea, enabling us to reconstruct glacial-
interglacial climate variability and past atmospheric circulation patterns from marine oxygen-isotope stage
(MIS) 15 to the last glacial period (MIS 2) . Presently located at the interface between Mediterranean and
continental climates of central and eastern Europe, the loess record of Dobrogea offers insight into long-term
paleoenvironmental oscillations triggered by the reciprocity of Mediterranean and continental atmospheric
circulation patterns across central and eastern Europe.
The 35m thick loess sequence at Mircea Voda shows a well exposed sequence of loess-paleosol couplets
that can be traced laterally across a few hundred meters, suggesting a semi-continuous paleoclimate record
since MIS 15. In order to assess the loess record of aeolian dynamics and associated past-synoptic
atmospheric circulation modes, high resolution particle size analyses have been carried out using a
Beckman-Coulter LS 13-320 laser analyzer.
With support of amino acid geochronology data, as well as
sedimentological features noted in the field, the highly resolved particle-size record of the Mircea Voda loess
sequence reveals clear shifts in the aeolian dust record and a general paleoclimatic trend from subtropical
(MIS 15) to more continental climates (MIS 1). The observed long trends in the aeolian dust transport record
and the general tendency of a progressive aridification since the Middle Pleistocene may be related to
interactions and/or shifts of the European polar front and the subtropical jet stream affecting the climate of
the Black Sea region on seasonal as well as geological time scales. The data from the Mircea Voda loess
profile offer the potential to link continental climate records of SE-Europe with paleoclimate archives of the
Black Sea region in order to decipher eastern Mediterranean climate variability.
PP13A-1425
Amino Acid Geochronology as an Independent Test of Numerical Dating Methods Applied to Loess Deposits in Southeastern Europe and Central Asia
Paleoclimatic investigations of loess-paleosol sequences depend on the application of numerical dating techniques, including radiocarbon, luminescence, and paleomagnetic methods, in order to develop reliable time series for the proxies being studied. Commonly, the utility of luminescence and radiocarbon dating is limited by their applicable dating range. At many European and Central Asian loess sites, luminescence results show a significant age underestimation for samples taken from loess of the last glacial cycle. Ages of lower to middle Pleistocene strata are limited to relatively coarse estimates using biostratigraphy and paleomagnetic reversal stratigraphy. Amino acid racemization (AAR) geochronology, although primarily a relative dating method, offers an independent assessment of numerical age estimates when results are at or near their methodological limits and can assist in the chronostratigraphic evaluation of loess units beyond the applicable range of numerical dating methods. In this study we present the first comprehensive aminostratigraphic results from a Central Asian loess site: the Remisowka loess sequence in southeastern Kazakhstan. We also present new results from the loess series at Stari Slankamen and the Mosorin (Titel) Plateau in Vojvodina, Serbia, and offer initial results from the loess sequence at Mircea Voda, Romania. We measured D/L ratios of glutamic acid, aspartic acid, phenylalanine, valine, and alloIsoleucine/Isoleucine in fossil shells of the terrestrial gastropod genera Pupilla, Pseudonapaeus, Leucozonella, Helicopsis, Vallonia, Clausiliidae and Succinea, which were selected for their wide geographic occurrence and reliability and reproducibility in repeated D/L measurements. The results of AAR analyses allow us to propose an independent age model for the studied loess series, and their combination with numerical dating techniques allows us to confidently determine a chronostratigraphic model for the last four interglacial-glacial cycles in Serbian, Romanian and Central Asian loess sequences, casting new light on the Middle to Upper Pleistocene chronostratigraphy and paleoclimate record of Eurasian loess. At Stari Slankamen, AAR data identified an unconformity in the second loess unit, representing a previously unrecognized missing interglacial paleosol, leading us to propose a new chronostratigraphic interpretation for that long-studied loess profile. In conjunction with other analytical and geochronological techniques, we aim to highlight the vast potential of highly resolved multi-proxy investigations of the terrestrial aeolian dust record. As loess sediments are widespread on the continents and offer semi-continuous palaeoclimatic reconstructions on regional and inter- hemispheric scales, we anticipate that the implementation of new and refined methods will lead to a renaissance in the study of these important terrestrial climate archives.
PP13A-1426
Inter-hemispheric linkages and teleconnections of past atmospheric circulation and aeolian dust dynamics recorded in loess sequences across Eurasia
The distribution of Eurasian loess deposits allows interregional palaeoclimatic investigations along a west-
east transect across the entire Eurasian loess belt of the Northern Hemisphere, offering the potential to
reconstruct Pleistocene atmospheric circulation patterns and aeolian dust dynamics on a wide spatial scale.
High resolution particle size analyses from several loess sequences across Eurasia (Serbia, Romania,
Uzbekistan and Kazakhstan) provide a detailed signal of glacial-interglacial atmospheric dynamics and long
term, semi-continuous trends in the aeolian dust record since marine isotope stage 10. In consideration of
the modern synoptic atmospheric circulation patterns and aeolian dust transport across the Eurasian
landmass, we propose that the observed proxy data reflect oscillations superimposed on a long term signal of
seasonality, triggered by changes in duration and permanency of the seasonal shift of the Eurasian polar
front during the middle to late Pleistocene. As the activity of the polar front jet is intimately connected with the
high level planetary frontal zone (HPFZ), the Eurasian loess archives may also serve as a recorder of inter-
hemispheric climate connections in past atmospheric circulation. Although there are large scale similarities in
the dust transport record from numerous sites across Eurasia, the data reveal distinct differences in climate
variability along the studied transect from SE Europe to Central Asia. For example, the occurrence and
configuration of the last glacial maximum (MIS 2 / MIS 4) varies along the transect. This suggests that the
record contains distinct background signals of aeolian dust dynamics, such as a decreasing influence of
western Atlantic teleconnection influence from the west to the east, as well as temporal variation in the onset
of the last glacial cooling and its associated increase of aeolian dust activity.
http://www.agu.org/journals/gc/gc0808/2007GC001938/
PP13A-1427
Atmospheric circulation patterns and geochemistry time series from ice/firn cores and snow samples of central Asian glaciers (Pamir, Tien Shan and Altai).
Combination of high mountain ice-core isotope-geochemistry, ground based aerosol monitoring, NASA
remote sensed and a NOAA atmospheric pressure distribution data were used to receive information on
sources of dust/loess transport, their time and spatial extension in modern and pre-industrial time. Hundreds
of samples from snow pits and ice/firn cores obtained from central Asian glaciers were collected, processed
and analyzed. The NASA RS products address the gap in interpretation of available snow, firn and ice
records by providing the spatial resolution necessary for identifying possible local and regional-scale dust
sources, transport routes and depositions. NOAA Hypslit program modeled the air back-trajectories allowed
to found association between the ice core geochemistry records and aerosol sources. To find the circulation
patterns, which are closely associated with geochemistry ice core/snow pit records, the correlation
coefficients between the Empirical Orthogonal Functions of the atmospheric circulation patterns and
geochemistry time coefficients for first two unrotated scores were computed. The loess / dust storm sources
with corresponding geo-chemical composition (trace elements, major ions and dust particles) in western,
central and northern Asia were identified:
1. Tajik loess deposition and Iran, Afghanistan /Turkmenistan sands are for the Pamir. For example, the
Pamir ice core records that associated with Tajikistan loess deposition are characterized by high
concentrations of REEs and Al, high or median content of Ca, and a background S concentration. Samples
from the Pamir Mountains differed in having low concentrations of Gadolinium. Occasional intrusions of
Chinese loess to Pamir glaciers are not excluded. REE profile of pilot Pamir cores documented one of the
most extreme droughts of 2001 and 2002 that developed in south-west Asia.
2. Chinese loess deposition in the Takla Makhan, sands in the Tajikistan Deserts and western Gobi, and
dust aerosols from Aral region are for the Tien Shan;
3. Western and central Gobi and Kazakhstan dust are for the Altai. The Aral region is the source of dust
aerosol for Altai also. Based on in-situ data in the western Central Asia and Altai ice core records of particle
number content there is tendency on decrease of dust storms frequency from the 30th in central and eastern
Asia.
http://www.sci.uidaho.edu/cae/index.html
PP13A-1428
Semiquantitave Characterization of Dynamic Response of Long Records Paleoclimate Records.
Long paleoclimate records provide evidence for nonlinear changes in the climate system, yet the resolution of such records is considered too poor for many new mathematical techniques of phase space reconstruction and parameterization. In this presentation the author demonstrates that the past dynamic behavior of the climate system can be characterized semi-quantitatively. The deep ocean isotopic record and the record of magnetic susceptibility of loess both provide long records of climate change. Reconstructed phase space portraits of the latter part of both records are very similar and suggest that climate in the late Quaternary has been characterized by rapid modal changes between up to four separate metastable equilibria. Prior to the mid-Quaternary transition, the deep-ocean isotopic record shows modal changes between no more than three metastable equilibria, but the loess record displays dramatically different behavior, which may be simple chaos. The reasons for the observed change at the mid-Quaternary transition are unclear, but are consistent with a model in which climate fluctuation is driven at the smallest scales by linear response to variations in Earth orbital parameters, but large-magnitude changes to occur when local forcing drives the system beyond some threshold.
PP13A-1429
The Alleret Maar lacustrine sequence (French Massif Central): a 150 ka long early-middle Pleistocene continental paleoenvironmental record.
Lacustrine maar sequences of the French Massif Central are of great interest for paleoclimatic and paleoenvironmental reconstructions of mid-latitudes Quaternary continental environments. In particular, the western Velay region yields exceptional sequences spanning the last 450 ka (Reille et al., J. Quat. Sci. 2000). However, older sequences remain largely unknown despite the presence of interbedded alkaline tephras allowing precise absolute radiochronological control of many lacustrine squences. The Alleret maar is a 1500 m wide phreatomagmatic crater that provides a long lacustrine sequence (41 m). The upper part of this sequence (AL2 core, 14.6 m) was studied between 2005 and 2006 (Pastre et al., C. R. Acad Sci, 2007). A 39Ar/40Ar date (557 ± 5ka) obtained from an interbedded tephra layer located at 7m as well as the associated pollen data attribute the beginning of this sequence to the MIS 15. Thanks to the AL3 core recovered in 2005 (40.6 m, CNRS Meudon) several new tephra layers were discovered in the bottom part of this lacustrine sequence. Three new 39Ar/40Ar ages (single crystal analyses) from trachytic tephra layers were obtained at the LSCE Argon Laboratory (France). These layers are located at -30.2, -36.2 and -39.2m. Ages obtained relative to the ACR-2 flux standard (1,201Ma, Kuiper et al., Science, 2008) range from 692 ± 6 ka (MSWD: 2.3, n=18) for the youngest (-30.2m) to 726 ± 9Ka Ka (MSWD: 2.2, n=12) for the lowest tephra located at -39.2m. These new dates indicate a relatively homogeneous deposition rate of 3.5cm/ka and that the last 10 meters cover the MIS 17-MIS18 period. According to these current radiochronological data the complete lacustrine sequence last more than 150ka. Ongoing sedimentary and pollen studies will allow to extend the paleoenvironmental and paleoclimatic records of the French Massif Central towards the beginning of the early middle Pleistocene.
PP13A-1430
Droughts in North Africa - New Insights From a 200 Year Record of Helium-4, a Proxy for Dust, in a Red Sea Coral
Mineral dust emitted from arid and semi-arid regions of North Africa is an important component of the Earth's climate and has been causally related to prolonged periods of drought in the Sudano-Sahel region in the later part of the 20th century. To quantitatively constrain factors responsible for the observed trends in dust activity and to understand the effect of future climate change on dust mobilization from Sudano-Sahel region, we have reconstructed a 200 years record of dust export using 4He as a tracer of dust in a fast growing coral (Porites lutea), collected in 1995 from the northern Red Sea. Our proxy record of dust emission rates from North Africa span the period from late 1790's to mid 1990's. The 200 year record of dust emission rates from North Africa indicates an approximately 12 year periodicity in major droughts in North Africa. High 4He fluxes correspond to all major droughts of the 20th century in North Africa e.g. in 1900's, 1910's, 1940's, 1970's and the 1980's. Although dust export rates from North Africa has been periodically high in the last 200 years, yet on an average, dust fluxes in the 19th century has been a factor of 2.9 higher than that in the 20th century. Dust emission rates from North Africa were exceptionally high at the terminal stages of the Little Ice Age (between 1830 and 1870). Comparisons between 4He record and other climatic factors affecting North African climatology indicate cooler tropical sea surface temperatures and enhanced solar activity to be associated with major droughts in North Africa. There is a remarkable co variation of dust emission rates with solar activity in the 19 th century. However, high solar activity is also present in the latter part of the 20th century (1956-1990) which coincides with the drying up of the Sahel. But dust emission rates associated with the late 20th century droughts is a factor of 3.1 lower than that associated with the droughts of the mid 19th century. We hypothesize that the magnitude of the late 20th century droughts may have been partially countered by rising tropical sea-surface temperatures. Overall, solar activity and tropical sea surface temperatures appear to have played major roles in generating droughts in North Africa in the last 200 years.
PP13A-1431
Using Helium Isotopes in marine sediments as tracers of continental inputs, provenance, and accumulation over the last 7Ma
Linking changes in Northern Hemisphere ice sheet growth and ocean circulation to 7 to 8Ma long Chinese Loess records are critical for understanding potential impacts of monsoon development, westerly circulation, aridity, and Tibetan Plateau uplift (e.g. Sun, et al. 2008). Here, we use the helium isotopic composition of marine sediments at two sites, IODP Site 1313 (41N, 32W at location of DSDP Site 607) from the North Atlantic and ODP Site 885/886 over the last 6.2 and 7 Ma, respectively. Helium isotope analyses provide estimates of detrital inputs from 4He (Patterson and Farley, 1999), MAR from 3He derived from interplanetary dust (Higgins, 2001, Marcantonio et al., 1996), and provenance changes from 3He/4He ratios. The distal eolian record of Rea et al. (1998) at ODP Site 885/886 (45N, 168E) suggests a large step-wise increase in dust at 3.6Ma that marked a change in aridity and plateau uplift. He isotope analyses by Higgins (2001 and this study) confirmed the quantity of eolian materials but proposed a more gradual rise in eolian accumulations at Site 885/886 since 7Ma. IODP Site 1313 stratigraphy can be tied directly to global benthic isotope records (Lisiecki and Raymo, 2005) for the last 6Ma and provides a distal view of detrital inputs from the N. hemisphere ice sheets via formation/overflow of NADW linked to glacier growth and sea level. 4He isotope and 3He/4He ratios results at Site 1313 (this study and Farley,1995) indicate a dramatic swing from a predominantly basaltic end member to one increasingly continental at 3.6 Ma. The 6 Ma record is punctuated by increases in continental sources at 4.8 to 5.2 Ma, 2.5 to 2.7 Ma, 1.8Ma, 1 to 1.2Ma, 0.5 to 0.6 Ma. K/Ar analyses on select samples supports these 3He/4He results. The timing and pattern of detrital sources/MAR to 885/886 and 1313 are very similar for the last 6 Ma. Combining atmospheric records from distal N. Pacific with this distal sediment transport record from N. Atlantic suggests a very tight coupling of N. Hemisphere glacier growth, sea level changes, ocean circulation and atmospheric changes with little time for significant lead/lags with identified periods of Tibetan uplift.