PP43C-01
Lake-Sediment Evidence for Variation in Mid-Holocene Climate and Vegetation in Western Massachusetts
A growing body of evidence indicates that significant, worldwide variations in climate occurred during the middle Holocene. Awareness of mid-Holocene climate change in eastern North America has implications for interpreting the vegetation history of the region. The range-wide decline of eastern hemlock (Tsuga canadensis) at 5400 cal yr BP has been interpreted by many as a pest or pathogen attack, though recent evidence attributes the event to an abrupt change in climate. A new sediment record from the Berkshire Hills of western Massachusetts, a region of historically high hemlock abundance, helps to address questions relating to the Holocene environmental and ecological history of eastern North America. Analysis of fossil pollen yields a record of changes in vegetation, while down-core variations in organic content (loss-on-ignition; LOI) serves as a proxy for changes in water depth and subsequently climate. Hemlock first arrived at Benson Pond 11,000 cal yr BP, when it abruptly increased in abundance. Hemlock abundance was relatively high between 9000 and 6600 cal yr BP, though there was some variability, relatively low from 6600 cal yr BP to 5900 cal yr BP, and then very high from 5900 until the abrupt decline at 5400 cal yr BP. Abundance remained low until 3700 cal yr BP. Variations in LOI indicate that the 6600 and 5400 cal yr BP declines in hemlock may have been associated with dry climatic conditions. Intervals interpreted as being relatively dry include 13,000-11,600 cal yr BP (the Younger Dryas event), 9500-9000 cal yr BP, 8500-8300 cal yr BP (perhaps representing the 8200 cal yr BP event), 6600-6000 cal yr BP, 5400-4700 cal yr BP, and 3900-3700 cal yr BP. Increased abundance of aquatic taxa during some of these intervals provides further evidence for lower lake levels, in particular during the hemlock decline beginning at 5400 cal yr BP. These data suggest climatic control of eastern hemlock abundance throughout the Holocene, but further research on high-resolution records is needed to better understand the role of short-term, abrupt climatic variability on the vegetation history of eastern North America.
PP43C-02
Defining the Provenance Characteristics of Weddell Sea and Northeastern Antarctic Peninsula Sediment: A Multi-proxy Approach
The purpose of this study is to define the provenance characteristics of Weddell Sea and northeastern Antarctic Peninsula sediments, which will help identify past episodes of ice discharge from the Larsen and Ronne- Filchner Ice Shelf systems. Sampling targeted diamict where available; otherwise, sandy mud was collected. Samples were obtained from piston and phleger cores from the International Weddell Sea Oceanographic Expedition (IWSOE68, IWSOE69, IWSOE70), which were taken near the edge of the Ronne-Filchner ice shelf and along the west coast of Queen Maud Land. Jumbo piston cores and kasten cores from LMG05-02, NBP00-03, and NBP06-03 were collected from nearshore sites, in fjords, bays, and basins on the continental shelf. These sites were covered by the former Larsen-A and Larsen-B ice shelves until 1995 and 2002, respectively. The 500 μm-2mm grains were imbedded in epoxy for lithic fragment and mineral identification using optical microscopy. Preliminary observations in the Eastern Weddell Sea samples reveal the abundance of quartz, feldspars and sedimentary lithic fragments. Samples that were collected from the nearshore sites in the sub- Larsen area contain felsic intrusive lithic fragments, commonly in the form of quartz-rich granite. Microcline was present in a large portion of samples. Particle size analysis of samples from the edge of the Ronne-Filchner Ice Shelf showed a multi-modal distribution with no correlation to water depth. Samples were poorly sorted, suggesting contributions from ice-rafted debris. Particle size results from the Larsen-B Ice Shelf sites had very similar particle size distributions, displaying a weakly bimodal distribution with a large peak in the very fine to fine silt range and a smaller peak in the very coarse sand range. Work in progress includes major and trace element geochemical analysis of the <63 μm fraction, which will allow the study of fine-sediment distribution by ocean currents.
PP43C-03
Preliminary U-Th Dating of Endogenic Gypsum From Lake Petén-Itzá, Guatemala
Uranium-thorium dating of lake sediments can provide accurate ages beyond the range of radiocarbon dating. Many lakes have endogenic precipitation of calcium bearing minerals that allow for substitution by uranium within their crystal structures. The water chemistry of Lake Petén-Itzá, located in northern Guatemala, is dominated by calcium, sulfate and carbonate ions (Deevey et al. 1980; Curtis et al. 1998). Changes in regional climate due to suspected shifts in the ITCZ over the past 85,000 years resulted in both calcite and gypsum deposition, especially during drier periods (Hodell et al. 2006). Both precipitates incorporate uranium from the lake water and can be used for U-Th dating. U-Th dating is problematic in lake systems for several reasons: the potential for 1) diagenetic alteration or dissolution while the endogenic precipitates are settling and after deposition and 2) contamination by detrital thorium. Contamination is a primary issue because of the difficulty of separating the precipitates from the sediment. Despite these concerns, U-Th dating of lake sediments has the potential to be successful with careful selection of precipitates and thorough analysis of the precipitate, the lake system, and the diagenetic environment. Preliminary samples of Lake Petén-Itzá gypsum contain euhedral crystals, indicating a lack of post-depositional alteration. However, the samples have high background thorium levels, indicating the necessity of using the U-Th total sample dissolution isochron dating method (as described by Ku, 2000) to quantify authigenic thorium. Curtis J.H., Brenner M., Hodell D.A., Balser R.A., Islebe G.A., Hooghiemstra H. 1998. A multi-proxy study of Holocene environmental change in the Maya lowlands of Petén, Guatemala. Journal of Paleolimnology 19, 139-159. Deevey E.S., Brenner M., Flannery M.S., Yezdani G.H. 1980. Lakes Yaxha and Scanab, Petén, Guatemala: Limnology and hydrology. Arch. Hydrobiol. 57, 419-460. Hodell D.A., Anselmetti F.S., Brenner M., Ariztegui D. 2006. The Lake Petén-Itzá Scientific Drilling Project. Scientific Drilling 3, 25-29. Ku T-L. 2000. Uranium series methods: in Quaternary Geology: Methods and Applications. AGU Reference Shelf 4, 101-114.
PP43C-04
Climate Change during the Last Deglaciation in the lowlands of Central America
In Feb-March 2006, we recovered 1,327 m of sediment core at seven sites in Lake Peten Itza, northern Guatemala. Preliminary post-drilling research has focused on Site PI-6 located in a water depth of 71 m where three holes were drilled to a maximum depth of 75.9 mblf. Using an ITRAX XRF core scanner at the University of Minnesota - Duluth, we measured a suite of elements between Al and Ba (e.g., Al, Si, P, S, K, Ca, Ti, Mn, Fe, Rb, Sr, Ba) at 0.5-cm resolution from 9.85 to 25.92 mcd in the spliced composite section at Site PI-6. This interval spans the period from the Last Glacial Maximum (24 kyrs ) to just above the Pleistocene/Holocene boundary (~8.3 kyrs). The chronology was derived by linear interpolation between 7 well-ordered radiocarbon dates on terrestrial organic matter. Elemental variations largely reflect relative changes in sediment composition between three dominant components: sulfates, carbonates, and silicates. Sediments deposited during the last glacial maximum (LGM) from ~23 to 18 kyrs were marked by high values of detrital elements (K, Rb, Fe, Ti) reflecting clay deposition in deep water under relatively wet climate conditions. This finding contradicts previous results suggesting that the LGM was dry in the Peten lowlands. From ~18 to 14.7 kyrs, sulfur content is high reflecting deposition of gypsum under arid climate. Within this generally dry interval, a period of very rapidly fluctuating gypsum (S) and clay (K, Rb, Fe, Ti) interbeds occurred between 16 and 15.5 kyrs. Coincident with the start of the Bolling-Allerod at 14.5 kyrs, gypsum deposition ceased and detrital elements (K, Rb, Fe, Ti) increased abruptly and remained relatively high between ~14.7 and 12.8 kyrs, indicating wetter conditions. At the start of the Younger Dryas at 12.8 kyrs, detrital elements (K, Rb, Fe, Ti) declined and S increased reflecting gypsum deposition. Relatively dry conditions persisted until ~10.2 kyr when gypsum deposition ceased and mesic conditions prevailed in the early Holocene. This inferred deglacial sequence from Lake Peten Itza in the lowlands of Central America will be compared with records of ITCZ migration inferred from the Caribbean (Cariaco Basin), meltwater input to the Gulf of Mexico, and polar ice cores from Greenland and Antarctica.
PP43C-05
Ostracode Paleoecology and Trace Element Shell Chemistry: A case study from Laguna Salada, Baja California, Mexico.
The Laguna Salada Basin in northeastern Baja California constitutes the southwestern limit of the Salton Trough and provides insights into the evolution of marine and non-marine sedimentary basins during the latest Neogene at the head of the Gulf of California. A 43 m long core (LS04-1) recovered from the Basin consisted of alternating clay, silt and sand beds with gypsum lenses. Three radiocarbon dates provided the age control: 41,520±790 cal years BP at 30.21 m, 25,590±320 cal years BP at 26.60 m and 15,150±70 cal years BP at 7.50 m. A total of 103 samples were analyzed for taxonomy, taphonomy and geochemistry. Eighteen marine and brackish- water ostracode species and nine benthic foraminifera species were identified. Faunal changes suggest variations in the water source with the opening of the basin to the Gulf of California. The Mg/Ca and Sr/Ca ratios of valves of two ostracode species (Limnocythere staplini and Cyprideis castus) were measured using ICPMS. Based on the Mg/Ca ratios, water paleotemperatures were estimated using a regression model. Shell chemistry shows mixed results. Some intervals fit an acceptable range, whereas other are unrealistic. Analysis of Sr/Ca ratios ostracode valves from the conflicting horizons show evidence of diagenetic effects. The fossil assemblages indicate two dry periods: prior to the 41,520 cal years BP and after the 15 ,150 cal years BP, with the maximum highstand of the basin between 25,590 and 15,150 cal years BP. Given the diagenetic effects at Laguna Salada, ostracode shell chemistry paleotemperature estimates may not be warranted in attempting to determine this parameter from the dry periods shown by faunal assemblages.
PP43C-06
Evidence of two glacial-interglacial climatic shifts from lacustrine records of TOC, δ13C and C/N ratios in the southwestern USA
Valles Caldera, located in Northern New Mexico, contains lacustrine deposits dating back to the middle Pleistocene. An 82 m long core containing ~75 meters of lacustrine sediments was recovered from Valles Caldera in 2004. The age of the sediments was constrained by Ar-Ar dating of a primary tephra layer from the base of the core, in combination with correlation to the deep-sea marine isotope stage record along the length of the core. The lacustrine sediments span from ~350 k YBP to 552 k YBP covering two glacial-interglacial stages, from MIS 11 to MIS 14 and part of MIS 10. Total organic carbon (TOC) in the siderite free fraction (i.e. diagenetic siderite was chemically removed) ranged from <1% to 7% with the lowest values predominantly observed during glacial times and marked increases at the onset of interglacials. Particularly high TOC is observed during the long interglacial MIS 11. C/N ratios ranged from values of <2 to ~11 with lowest values in general observed during glacial times indicating increased input from aquatic sources and suggesting deepening of the lake as a result of wetter glacial climate. δ13C values measured in the siderite free fraction, ranged from - 28‰ to -20‰ and remained more negative overall during glacial times. Further isotopic analyses including δD values of n-alkanes will be performed to reconstruct the delivery of meteoric water to the region and thus contribute to understand the history of climate change.
PP43C-07
Band-counting and grey tone spectral analysis in banded stalagmites, a new proxy for paleoclimate reconstruction?
Stalagmites are considered one of the most reliable records of past climate variability, due to their evolution in closed-system conditions, ease of dating, and inclusion of several geochemical proxies (such as calcite oxygen and carbon isotopic composition, trace element concentration and/or elemental ratios). While these have been proven as very useful to extract paleoclimatic information, they are also labor intensive and expensive. Banded stalagmites represent an opportunity to assess whether variations in stalagmite color can be related to climate variations. This is because stalagmite color is related to the amount of detritus and organic material included at the time of calcite precipitation. Thus, analysis of high-resolution images of banded stalagmites for variations in grey tonality might represent an additional proxy for paleoclimate reconstruction. Here we present the first results from the image and spectral analysis of a stalagmite from cueva Los Riscos in northern Querétaro, Mexico. The stalagmite has been dated using U-series disequilibria, and band counting was carried out using a band recognition algorithm. The results from the spectral analysis are discussed within the global climatic context and compared to other paleoclimatic records.
PP43C-08
Late Holocene Droughts Recorded in Speleothems From the Maya Lowlands of the Yucatan Peninsula, Mexico
Evidence for Holocene droughts in Mesoamerica exists from numerous paleolimnological studies. Speleothems from the Yucatan Peninsula can be used to help constrain the timing, intensity, and regional extent of these droughts. Late Holocene wet/dry cycles are inferred from the oxygen isotope record of speleothem calcite and the timing is constrained using U/Th dating. In the lowland neotropics, there is a strong negative correlation between δ18O of precipitation and rainfall amount, i.e. the "amount effect." The δ18O of speleothem calcite can then be used as a proxy for the relative amount of past precipitation in the Maya lowlands. A speleothem from Cueva Tzabnah, near Tecoh, Yucatan, Mexico, has a basal date of 2711 ±329 yr BP and a top date of 548 ±61 yr BP, indicating growth during most of the period of Maya prehistory. The 25cm-long speleothem has an average growth rate of 0.12mm/yr, and contains several visible hiatuses indicating cessation of growth, possibly due to drier climate conditions. Oxygen isotopes were measured at 0.5mm intervals, with an average value of -5.24‰. δ18O values increase near the hiatuses in speleothem growth, interpreted as evidence for the presence of drought conditions. These relative increases in δ18O will be dated using U/Th methods to constrain the timing of the drought events. In the top 3mm, the average δ18O value is 2.49‰ greater than the average speleothem δ18O value, and possibly represents drier conditions beginning in the mid-15th century AD consistent with a nearby lake sediment core record from Aguada X'camaal (Hodell et al., 2005). Well-dated drought events during the late Holocene will provide insight into the role that climate may have played in the cultural evolution of the Maya civilization. References: Hodell, D.A., Brenner, M., Curtis, J.H., Medina-Gonzalez, R., Ildefonso-Chan Can, E., Albornaz-Pat, A., Guilderson, T.P., 2005. Climate change on the Yucatan Peninsula during the Little Ice Age. Quaternary Research 63, 109-121.
PP43C-09
Comparison of new geochemical proxies in a stalagmite from sub tropical Mexico
The mexican subtropics represent an interesting area to understand past climate change and variability since it is located within the current northern limit of the Inter Tropical Convergence Zone(ITCZ). Consequently, the area is very sensitive to changes in the average position of the ITCZ. We have collected a 33cm stalagmite from Cuetzalan, Pue. Mex (20°00'N, 97°32'W), that grew between 24.08 to 22.35 ky B.P. The stalagmite was analyzed for Sr/Ca, Ba/Ca, Mg/Ca, U/Ca molar ratios, δ13C, δ18O, [234U/238U], and other trace elements, and represents the first stalagmite record from tropical Mexico. Significant variations in the [234U/238U] in the stalagmite, suggest changes in the local weathering rates that are not reflected in other geochemical proxies. However, comparison of the E/Ca and δ18O with records from Hulu (China) and Botuvera (Brazil) caves suggest that the climate in the area was in line with the global climate system.
PP43C-10
Diet and habitat landscape of Equus sp. and Mammuthus columbi for the Late Pleistocene deposit at El Cedral, San Luis Potosi, Mexico
The diet and habitat landscape for fossil specimens pertaining to Late Pleistocene horse Equus sp., and mammoth Mammuthus columbi from the deposit at Rancho La Amapola, El Cedral, San Luís Potosí, México were assayed with basis on stable isotopes analysis the carbon and oxygen found in dental enamel apatite. For horses the Δ 13C values showed from C3/C4 a basic C4-plants diet ranging into a C3/C4 mixer feeder, while mammoth showed values as a C3/C4 mixer feeder, but preferring C4 plants; there was not significant differences among the average values for both groups, with -3.5 ‰ for horses and -3.8 ‰ for mammoth. Mexican data was compared with those for the same genera from US localities, including the states of Florida, New Mexico, and Texas, allowing confirmation of the inference that El Cedral species were mainly grazers, but with some C3 plants being consumed; furthermore, comparisons of El Cedral specimens, using both carbon and oxygen isotopes, with Floridan C3-plant sites showed El Cedral specimens could be defined as mixer feeders in open habitats, preferring such opn areas like grassland or savanna. Our results are in agreement with the species dental morphology, as well as with the pollen record for the same site, which showed the existence of grassland that could have extended during the Late Pleistocene from southern Canada to central Mé xico
PP43C-11
Biogeochemical Controls on Paleoproductivity Proxies: The Antarctic Sea Ice Environment
The Southern Ocean features prominently in many hypotheses developed to explain the drop in atmospheric
CO2 during Quaternary glaciations. Critical to the testing of these hypotheses is our ability to reconstruct
past conditions of Southern Ocean primary productivity, nutrient availability and utilisation; based on isotopic and
trace element proxies. One aspect of these proxy records not fully investigated is the extent to which they are
influenced by materials formed within sea ice, an environment vastly different from that of the open water column.
Most proxies are calibrated for open ocean settings, so material produced within sea ice could potentially
confound proxy-based reconstructions of water column conditions. Detailed studies on sea ice material are
lacking for most isotopic and trace element proxies used in the Southern Ocean.
We report initial results from time-series studies of sea ice and surface water suspended particulate matter in
Ryder Bay (67°S, 68°W). Preliminary results show differences in productivity (chl-a, POC/PON) and
the isotopic (δ13C, δ15N) and trace element (Ba) composition of nutrients and organic
matter formed in sea ice brine and surface waters. Our sampling spans the winter sea ice cover, transition to
open water and the spring bloom over two field seasons (2004/5 and 2005/6). Additionally, through similar
analysis of materials collected in sediment traps deployed at this site, the extent to which geochemical signals
produced in the sea ice environment influence the sedimentary record was investigated. This proxy data was
compared to measured environmental parameters including; temperature, the dissolved CO2 system,
nutrient utilisation and dissolved oxygen to allow quantitative analysis of proxy signatures relative to observed
surface water conditions. This information was then used to apply sea ice related proxy signals to sediment
records from the Antarctic sea ice environment.
http:www.dsc-ed.org.uk
PP43C-12
Implications of Climate Change and Tephra Accumulation on the Lacustrine Environment at an Archaeological Site in Michoacan, Mexico
La Alberca, a volcanic caldera in the active Michoacan-Guanajuato volcanic field of central Mexico, is a site of ancient occupation. The oldest known burial in the state of Michoacan, approximately 5000 years old, was discovered immediately beneath prehispanic cliff paintings on the caldera wall. The site is surrounded by numerous monogenetic cinder cone volcanoes, some of which may be the source of the tephra found throughout the site. The fluvial and lacustrine sediments in the base of the caldera exhibit a bimodal sedimentation pattern of coarse, sandy tephra layers alternating with finer sediments that accumulated in frequent, shallow pools. Driving factors are the timing of local volcanic activity and variations in regional climate. The two deepest trenches contain zones of coarse, dark tephra and cobbles at the base, overlain by a 2-m sequence dominated by laminated or bioturbated fine sand, silt and clay sediments from 2360-530 years BP. A sharp contact divides this middle zone from the upper 2 meters, which is dominated by multiple coarse tephra layers that represent both direct airfall and redeposition within the watershed. Paricutin volcano is the source of some or all of the upper tephra layers. Changes in the sedimentation patterns at La Alberca were compared to regional paleoclimatic reconstructions from nearby lake basins. The zone of fine-grained, laminated sediments in the middle section of the stratigraphy corresponds with drier climatic conditions in the surrounding region of central Mexico, but may represent longer persistence of ephemeral lakes in the lowest portion of the caldera basin because the sediments were less permeable. The zones with a greater influx of coarse tephra correspond in time with wetter climatic periods that may have mobilized the tephra that blankets the watershed. Permeability of the surface substrate increased, likely decreasing the duration of standing water. These variations in the lacustrine and geomorphic environment of the basin could have implications for the types of human uses of the archaeological site within the caldera. Geochemical analyses of the tephras at this site will aid in dating and correlating sediments, determining sources of the tephras and understanding the impact that the influx of tephra has on the lacustrine and fluvial environmental changes in La Alberca. Electron microprobe analysis of a massive, coarse tephra unit beneath the burial in the rockshelter at La Alberca revealed a distinctive, low-silica content geochemistry that will help in its identification and enable a direct link to be made between the archaeological deposit at the rock shelter and the broader paleoenvironment. Additional analyses will be used to directly compare the timing of the stratigraphic variations in the caldera basin with the human uses of La Alberca rock shelter and other archaeological sites in the vicinity of the caldera.
PP43C-13
Palaeoenvironmental Fluctuations Identified by Diatoms, Ostracode, Geochemistry and Sedimentary Facies from a Tropical Crater Lake on Western Mexico
A ca. 2500 cal. yr record from Santa Maria del Oro (SMO) was studied for its diatoms, ostracodes, magnetic mineralogy, geochemestry, organic matter and carbonate content to document climatic and limnological trends over western Mexico, a site sensitive to the intensity of monsoon circulation and to any latitudinal shifts of the high pressure subtropical cells as it is currently located on a transition zone between the temperate climates of central Mexico highlands and the arid climates of northern Mexico. The sediment sequence (8.8 m) has a chronology is based on 7 AMS dates. The sequence shows an irregular sequence of four distinctive facies (laminations). Data derived from the analysis of easch facies and of modern sediments supports the interpretation that sand and peaty layers are mostly alloctonous in origin while the silty layers (ochre, brown and greenish silt) are mostly autigenic. Ochre layers represent times of anoxic bottom lake conditions favoured by a warmer environment. Long term trends are also present along the sequence. Warm and relatively humid conditions change to a drier climate by ca. AD 600, with the driest and warmest stage recorded at AD 990. This dry period is consistent with records of arid phases at other sites in Mesoamerica. From AD 990 to AD 1260 there is evidence of a sudden transition to warm-humid conditions and dilution of lake water coeval with the Medieval Warm Period. From AD 1260 to AD 1480 climatic conditions were unstable and afterwards the onset of drier climates is recorded until the present. Due to its geographical position, moisture availability fluctuations are responding to atmospheric circulation changes such as latitudinal shifts of the Intertropical Convergence Zone and Subtropical High- Pressure Cells resulting in the strengthening or weakening of the summer monsoon, which is the main moisture source for western Mexico. This sequence records environmental and climatic variability for western central Mexico and extends the geographical range of paleoclimatic records for this latitude in North America.
PP43C-14
Paleoclimatic changes between ca.2 and 27 ka cal BP derived from lacustrine sediments of Laguna San Felipe, Sonoran Desert, NW Mexico
Laguna San Felipe (115° 15' W, 31° 08' N) is located in the western part of the Sonoran Desert (rainfall 100-200 mm/yr), NW Mexico. Multi-proxy analysis in the upper 5.6 m sediments from a 9.5 m core of the laguna, including sedimentology, mineralogy, multiple element geochemistry, Corg, diatoms and mineral magnetism, were performed to document the late Quaternary climatic changes of the Sonoran Desert. AMS 14C ages constrained the sediment profile between ca.2 ka cal BP and ca.27 ka cal BP. The distribution of mineralogy, major and trace elements and magnetic susceptibility along the sedimentary sequence document the varying terrigenous influx, chemical weathering, precipitation/evaporation and hence climatic changes during last glacial-post glacial transition. The phases of higher evaporation in the basin and lower chemical weathering in the catchments indicate dry conditions during ca.27-14.5 ka cal BP, ca.13-12 ka cal BP and ca.7 ka cal BP. During these arid phases, the higher influx of both quartz and zircon into the basin could be due to the increased aeolian activity in the surroundings of the laguna. The phases of higher chemical weathering suggest that the laguna received higher inflow during ca.14-13 ka cal BP, ca.12-8.5 ka cal BP and ca.6-3 ka cal BP. During these phases, the preservation of Corg indicates anoxic condition and hence relatively high lake stand for part of the year.
PP43C-15
Natural Environmental Hazards Reflected in High-Altitude Patagonian Lake Sediments (Lake Caviahue, Argentina)
Two 6-m long sediment cores drilled in the two basins of Lake Caviahue give new evidence of the impact of natural hazards such as ash fallouts linked to nearby volcanic eruptions in the ecologically sensitive environment of the high-altitude region of the Argentinan Patagonian Andes. The two cores show distinct signals of changes in autochthonous productivity and terrigenous input into the lake from ash fallout, river load and shore erosion. Multiproxy records of the sediments indicate whether these changes can be related to volcanic activity. High values of magnetic susceptibility in the cores reflect periods of basaltic ash fallouts during eruptions of the nearby Copahue volcano. The southern basin is located in the prevalent direction of ash fallouts and has been affected by these volcanic inputs more intensely than the northern basin of the lake. In contrast, sedimentation and authochthonous productivity in the northern basin are strongly affected by fluvial inputs such as suspended river load and acidic stream waters.
PP43C-16
Reconstructing Tropical Sea Surface Temperatures Using Precipitation Proxy Records: Methods and Uncertainties
Reconstruction of historical tropical sea surface temperatures (SSTs) over the last millennia and beyond is an active venture in current paleoclimate research. This study explores the potential of using tropical precipitation data from paleoclimate proxy locations to reconstruct tropical ocean SSTs. The goal is to quantify the range of uncertainties in the SST reconstruction and their dependence on both errors in the proxy data and distribution of the proxy network. Two reconstruction methods are presented and applied to observational and modeled datasets over the period 1950-2000. The first reconstruction method exploits the high correlation between the leading mode of variability of precipitation and SST, which corresponds to ENSO and accounts for more than 50% of the total covariance. The second method uses multiple modes of covariability between precipitation and SST. We find that in addition to the ENSO mode, the second mode of covariability (about 20% of the total covariance) captures variations in the spatial expression of ENSO. This second mode has a large-scale spatial signature and is robust in all the precipitation datasets used in this study. Inclusion of this mode in the linear model leads to an approximate 30% improvement in the overall reconstruction skill. Uncertainty estimates computed from the linear model are consistent with those obtained using Monte Carlo simulations. Given the few degrees of freedom in the covariability modes, we find that a relatively sparse network of individual precipitation time series captures the phase of the modes and leads to high SST reconstruction skills in the tropical Pacific and Indian Ocean.