PP13E-01
High-Resolution Reconstructions of Temperature and Precipitation During the Last Millennium from Lake Tanganyika, Africa
Though numerous syntheses of high-resolution paleoclimate data have documented temperature changes associated with the Medieval Period, the Little Ice Age, and the Industrial era in north temperate regions, few records of temperature variability exist for the Southern Hemisphere (Mann and Jones, 2003, GRL doi: 10.1029/2003GL017814). Here, we present a new, high-resolution record of terrestrial temperature and rainfall from southeast tropical Africa, based upon organic geochemical analyses of annually laminated sediment cores from Lake Tanganyika, East Africa, providing the first millennium-long, high-resolution record of temperature from the East African tropics. Extending to 700CE with decadal resolution, the record uses the TEX86 paleothermometer and hydrogen stable isotope ratios (dD) of leaf waxes, which have been shown to reflect temperature and hydrologic conditions in Tanganyika, respectively (Tierney 2008, Science). The TEX86-based temperature reconstruction shows that Tanganyika temperatures were relatively warm at 700CE, cooler from 800-1000CE, and then warmer again between approximately 1050-1300CE. The latter warm interval is coincident with periods of warmer temperatures documented in the Northern Hemisphere. Yet temperatures are highly variable within the Little Ice Age (~1450-1800CE) and do not cool substantially relative to older time periods, in contrast to north temperate regions. Pronounced warming from approximately 1850 to the present is unprecedented in comparison to patterns of temperature variability during the previous centuries. The temperature variability documented here does not seem to correspond strongly to lake level fluctuations in Tanganyika over the past millennium (Cohen et al., 2005, JoPL, doi 10.1007/s10933-005- 2422-4), suggesting that Southeast African climate variability is more complex than alternating cycles of warm/wet and cool/dry conditions. dD analyses document considerable hydrologic variability over the past millennium, with temporal patterns that may be consistent with previous precipitation reconstructions from central East Africa. Overall, our records show that climate, particularly temperature, has changed abruptly during the past thousand years in southeast tropical Africa.
PP13E-02
High resolution records of rainfall changes over Papua New Guinea throughout the last 2000 years
The role of the western Pacific warm pool in modulating global climatic changes is critical, notably at the interrannual time scale through the El Niño-Southern Oscillation phenomenon. Yet, past changes in ENSO variability throughout the last millenia are poorly constrained from marine archives. To reconstruct past changes in rainfall variability at the annual to decadal time-scale during the last 2000 years in this region, we investigated two sediment cores from the northern margin of Papua New Guinea, off the mouth of Sepik river (MD05-2917C2 and MD05-2918 sediment cores). Using a multi-proxy approach combining XRF- scanning at sub-centimetric scale, biogenic fluxes, stable isotopes analyses of planktonic foraminifera, charcoal and pollen countings, we document changes in both mean conditions (decadal scale), and in extreme events likely correlated to ENSO variability. During the Little Ice Age (LIA), decreased 18O planktonic foraminifera values are synchronous with higher Ti/Ca values. This pattern clearly indicates that increased terrigenous fluxes correspond to fresher surface waters off Papua, due to an intensification of terrestrial runoff during the LIA. Paradoxically higher charcoals fluxes in the sediments during this interval point toward an increase of extreme dry events, which we interpret as corresponding to more drastic El Niņo events over Papua during the LIA. During the Medieval period (from 900 to 1200 AD), we document an opposite pattern, with drier mean conditions (decreased terrestrial inputs), and a decrease in extreme events from the charcoal record. Our records shed some new light on the dynamics of the ENSO throughout the last millenium, and we will discuss the potential climatic forcings at play for the opposition between decadal mean state and shorter time scales.
PP13E-03
Solar Activity Induced Pronounced Temperature Oscillations During the Late Pleistocene to the Early Holocene in the Northeastern North America
Sensitivity of the Earth's climate to the solar activity has profound implications for modeling future climate changes, but is a subject of intense scientific debate. GCM modeling considering solar forcing can successfully simulate climatic variations in the past millennia such as the Little Ice Age (LIA), but volcanic forcing during the same time period can also yield similar climatic effect and confound interpretations. Here we present decadal to centennial resolution compound-specific hydrogen isotopic records from two lakes in the northeastern North America to investigate the relationship between solar activity and temperature changes during the late Pleistocene to early Holocene. Our temperature reconstructions from the two lakes 100 km apart in New England are highly consistent with each other and agree well with established general climatic scenarios. More importantly, our records contain centennial-scale cyclicities related to the solar cycles (88 and 232 yr), indicating strong links between the pronounced 1 to 2 degree temperature oscillations and solar activity during the late Pleistocene. Our results strongly support the presence of an internal amplification mechanism for the solar forcing that is capable of causing disproportionally large climatic responses with relative small changes in the incoming solar radiation.
PP13E-04
A ~2500 Year Sub-Decadal Oxygen-Isotope Record of Late Holocene Tropical South American Hydroclimate From Laguna Pumacocha, Peru
We present a ~2500-year, sub-decadally-resolved authigenic carbonate oxygen isotope (δ18O) record of tropical South American hydroclimatic variability from finely-laminated (varved) sediments preserved within Laguna Pumacocha, central Peru. Monthly surface water oxygen isotope ratio measurements between May 2006 and April 2008 demonstrate that lake water oxygen isotopic values track the seasonal precipitation cycle and lake-level changes, with lower δ18O values and high water levels during the warm Austral summer wet season (January-May) and higher δ18O values and lower water levels during the cool Austral winter dry season (September-December). Oxygen isotopic values of authigenic carbonates collected from littoral vegetation and sediment traps indicate that carbonate is precipitated in equilibrium with lake water and that the measured values are representative of annual average lake water δ18O (δ18Olw). Model and observational studies suggest that the δ18O of precipitation during the wet season is strongly influenced by tropical Pacific sea surface temperature anomalies (SSTA), largely through effects on the South American Summer Monsoon (SASM). In general, warm (cold) SSTA are associated with reduced (increased) SASM precipitation, warmer (cooler) temperatures, and higher (lower) precipitation δ18O values. This relationship is reflected in the monthly Pumacocha δ18Olw measurements with higher maximum δ18Olw values during the 2006/2007 El Niño (warm Pacific SSTA) and lower maximum δ18Olw values during the 2007/2008 La Niña (cool Pacific SSTA). Accordingly, two major events stand out in the Pumacocha sediment δ18O record, a period of relatively low δ18O values during the Little Ice Age (~1500 to 1800 AD) likely indicating cool, wet conditions, and an extended period of higher δ18O values coincident with the so-called European Medieval Warm Period (~980 to 1100 AD), likely reflecting warm, dry conditions.
PP13E-05
Modeling Surface Water Transport in the Central Pacific Ocean With 129I Records From Coral Skeletons
129I occurs naturally in extremely low abundance via cosmic ray interactions in the atmosphere as well as by spontaneous fission of uranium. Oceanic concentrations of 129I have risen by several orders of magnitude during the last half century largely from environmental pollution coming from several point-source nuclear fuel reprocessing plants. In the Pacific basin, much of the increase has apparently come from the Hanford Nuclear reprocessing plant in the United States, with iodine primarily arriving via the Columbia River. Coral skeletons preserve records of 129I concentration of the surface waters from which they were deposited, yielding records with annual resolution or better. We will present three such records from different locations in the Pacific Ocean: the Solomon Islands, Easter Island and Clipperton Atoll. For this study, drill cores from living massive coral skeletons of the species Porites Lobata were collected from these sites. 129I/127I values were measured using accelerator mass spectrometry (AMS) at the University of Arizona with an NEC 3 MV Pelletron accelerator. Results from the analysis of the corals will be compared to the distribution of other mixed-layer tracers (chloro-fluorocarbons and tritium) collected during the World Ocean Circulation Experiment cruises conducted between 1990 and 2002. The 129I/127I records observed in these corals will also be compared to tracer transit time calculations determined from a 20th century simulation of the GFDL coupled-climate passive-tracer model.
PP13E-06
A 1400-Year Sedimentary Record from the Pigmy Basin, Gulf of Mexico Reveals Strong Decadal-Scale Linkages Between Solar Variability, Gulf of Mexico Moisture Balance, and Hydrologic Conditions over the North American Continent
Understanding hydrologic conditions between the North American (NA) continent and the Gulf of Mexico (GOM) during the Late Holocene is critical to understanding current and future responses to natural and anthropogenic climate changes. This study focuses on a marine sedimentary record spanning the last 1400 years (determined from seven AMS 14C dates) and utilizes a high resolution approach to define intervals of varying continental inputs and to assess changes in the moisture balance (E/P) within the GOM over time-scales relevant to human societies, including the Little Ice Age (LIA) and the Medieval Warm Period (MWP). Results show multi-decadal episodes of increased terrestrial inputs to the GOM (wet conditions) centered at 400 and 1200 years BP. These episodes coincide with intervals of increased δ18O seawater indicative of more saline conditions recorded in the Pigmy Basin. Episodes of low continental inputs (dry conditions) at 150, 850 and 1400 years BP are associated with decreased salinity in the GOM. This sediment record shows that hydrologic variability recorded over the NA continent is directly dependent on the moisture balance (E/P) over the GOM. Interestingly, the LIA and MWP are not characterized by modal difference in hydrologic conditions between the GOM and NA, rather these time intervals are highly variable and include both wet and dry intervals. Additionally, a δ18O ostracod record from Lake Punta Laguna on the Yucatan Peninsula, Mexico shows a strong coherency with the Pigmy Basin record where wetter intervals in the Yucatan lake correlate with lower E/P in the GOM and a dry NA whereas the drier intervals in the lake are associated with higher E/P in the GOM and a wet NA. Spectral analysis on the terrestrial input record exhibits a power at about 200 years, suggesting that solar variability is strongly influencing hydrologic conditions in the GOM and on NA. A Δ14C record from tree-ring studies used as a proxy for solar variability shows strong correlations with the terrestrial input record, GOM δ18O record, and the Yucatan lake δ18O record. We hypothesize that intensification of the Bermuda High pressure system due to solar forcing influences meridional moisture flux from the GOM to the NA continent; i.e. increased evaporation over the GOM leads to enhanced precipitation over the NA continent. Changes in solar variability appear to control the atmosphere-ocean-continent interactions that regulate regional scale changes in hydrologic conditions between the GOM and the NA continent.
PP13E-07
Sedimentary Basin Structure of the Hadar Formation's Lacustrine-Dominated Depocenter (Ledi-Geraru, Afar, Ethiopia) and its Relevance for Investigating Hominin Paleoenvironments
Sedimentary sequences preserved in East African rift basins record the long-term response of past depositional environments to climatic and tectonic forcing. Motivations for recent field investigations at the Ledi-Geraru site, part of the greater Hadar sedimentary basin in the Afar region of Ethiopia, stem from a need to characterize local basin structure and expand and refine interpretations of the complex mid-late Pliocene history of local and regional-scale landscape change during a time of critical importance for understanding hominin evolution. Detailed geologic mapping (1:7,000), measured stratigraphic sections, and seismic reflection surveys provide the datasets necessary for basin evaluation. The Ledi-Geraru sedimentary sequence (>250m thick) exposes nearly the entirety of the hominin-bearing Hadar Formation of west- central Afar. Both primary unmodified lake deposits and intervals modified by subsequent subaerial exposure and pedogenesis are well-exposed. The lacustrine-dominated signature is indicated by the prevalence of laminated silty clays that contain leaf impressions, fish scales, and gastropod shells, undisturbed laminated diatomite and clays, and pedogenically modified diatomaceous silts. The sequence is generally flat lying, with low bedding dips ranging from 0-2° NNW to <1° NNE and minor NNW trending faults with <5 m vertical offset. Whereas coeval fluvio-lacustrine sediments associated with hominin and archaeological sites west of Ledi-Geraru (e.g., Hadar and Gona) are marked by comparatively slow and episodic sedimentation, sedimentation rates in the Ledi-Geraru sequence are extremely high and consistent, on the order of ~0.9-1.0mm/yr. Laterally extensive tephra marker beds and paleomagnetic records provide excellent age control for sedimentation rate estimates and correlation to nearby fossil-rich sequences. As the Hadar basin sediments preserve a rich paleoanthropologic and archaeological record, this work provides the geologic framework necessary for a proposed (2011) continental drilling effort to obtain a near-continuous, ultra-high resolution terrestrial record of past climate variability from multiple paleo-lake basins in East Africa, including the Ledi-Geraru. A seismic reflection survey was completed there in spring 2008, below the planned drilling site. Gently east-dipping coherent reflections interpreted to be from the Ledi-Geraru sedimentary sequence are imaged in the seismic data to at least 0.2 to 0.3 s (two-way travel time). Preliminary average velocities of about 2000 m/s suggest a sequence thickness of 200-300 m. Furthermore, there is no indication of large-offset faults or of buried basalt ridges that would disrupt or reduce the stratigraphic column available for coring. The anticipated cores from Ledi-Geraru should yield a high-resolution chronostratigraphic framework and paleoenvironmental record from >3.5 to 2.9Ma. Combined geologic and seismic evaluations of depositional sequences are central for evaluating the geometry, tectonic evolution, and stratigraphic history of basins and facilitate interpretations of the space-time progression of evolving paleosurfaces.
PP13E-08
Interannual climate variability recorded by late Miocene laminated gypsums: evidence for ENSO in the Mediterranean
It has been hypothesized recently that ENSO may become stuck in an El Nino mode during warmer climate conditions such as existed in the Pliocene and Miocene. We report here the results of geochemical and spectral analyses of laminated gypsums from the Messinian (late Miocene) of Italy that appear to record annual variations in the Mediterranean climate. High-resolution geochemical analyses are consistent with an annual dry-wet cycle as the cause of the laminations. Analysis of time series of laminae thicknesses and annually-tuned grayscale profiles reveal significant spectral peaks at frequencies that match sunspot cycles and ENSO, and to a lesser extent, NAO. We simulated the climate of the late Miocene using CCSM 1.4, a fully coupled ocean-atmosphere-land-sea ice general circulation model and found that there is indeed an ENSO-like oscillation in equatorial Pacific SST that has a teleconnection in the Mediterranean causing significant variations in precipitation; analysis of a 200 yr time series of the modeled Mediterranean precipitation variability reveals spectral peaks whose frequency matches the high-frequency peaks from the laminated gypsums (representing equivalent time durations). The agreement between model results and proxy data suggest that the warmer mean climatic state of the late Miocene was characterized by ENSO variability similar to today's cooler mean climatic state ENSO apparently operates over a broad range of mean climate states.