GC51C-1059 0800h
The Lake Peten-Itza (Guatemala) Drilling Project: Tracking Late Quaternary Environmental Change in the Lowland Neotropics
During the last glaciation, all shallow lake basins in the northern lowland Neotropics were dry because of increased aridity and/or lowered sea level Consequently, no long continous paleoclimate records have yet to be recovered from this climatically sensitive region. Detailed seismic surveys of Lake Peten-Itza have revealed that Lake Peten-Itza is deep (160 m) and possesses a thick ($>$100 m) stratigraphic record that extends back to at least Marine Isotope Stage 5. It is the only lake known to us from the region that is deep enough (160 m) to have held water continuously through the latest Pleistocene. We have therefore proposed this lake as a potential drilling target for the International Continental Drilling Project (ICDP). The objective of the Peten-Itza Drilling Project is to recover continuous, high-quality sediment cores and pore fluids that will be used to test hypotheses related to three broad scientific themes: 1.) Paleoclimatic history of the northern lowland Neotropics on orbital to suborbital time scales emphasizing the correlation of terrestrial-marine-ice core archives (e.g., Cariaco Basin, Greenland ice cores, etc.) 2.) Paleoecology and biogeography of the Maya tropical lowland forest including the history of vegetation change and disturbance by humans, climate change, and fire. 3.) Biogeochemical cycling in deep lake sediments emphasizing integrated studies of microbiology, geochemistry (interstitial waters), and mineral authigenesis/diagenesis. Six primary and four alternative drilling sites have been identified that fall along a depth transect extending from $\sim$30 m to near the deepest point ($\sim$150 m) in the lake. A sequence stratigraphic approach will be employed to constrain the vertical range of past lake level variations for glacial, interstadial, and interglacial stages during the late Pleistocene. The integration of seismic profiles and a depth transect of Kullenberg cores have proven to be an effective strategy for defining lake level during the last glacial period and reconstructing paleoclimate and paleoenvironmental change for the most recent glacial-to-interglacial transition. The basal age of the sections to be drilled is not certain but based on extrapolation of dates for the shallow seismic stratigraphic units, we estimate that deeper sequences should extend through the last interglacial (MIS 5). An international team of research scientists has been assembled to produce a comprehensive suite of paleoenvironmental and biogeochemical measurements on the cores and pore fluids recovered from Lake Peten-Itza. Details of the drilling strategy, site locations, and site survey data can be found on the Peten-Itza Drilling Project website.
http://plaza.ufl.edu/hodell
GC51C-1060 0800h
Patterned Ground in Wetlands of the Maya Lowlands: Anthropogenic and Natural Causes
We use geological and archaeological evidence to understand the formation of patterned ground in perennial and seasonal wetlands in the karst depressions of Belize and Guatemala. Some scholars have argued that these features are the remnants of ancient Maya wetland fields, chinampas, on which intensive cultivation produced food that could begin to nourish the extremely high population of the Late Classic (A.D. 550-850). Others have argued that these were natural features or that they represent landscape manipulation for rising sea level in the Preclassic (1000 B.C. -A.D. 250). We present the evidence for ancient intensive agriculture and natural landscape formation with multiple proxies: excavated field and canal features, artifacts, pollen, soil stratigraphy, and water chemistry. Evidence thus far suggests that many regional depressions have Preclassic (1200 BC to AD 200) or earlier paleosols, buried from 1-2 m by eroded soils induced by Maya land use practices. These paleosols were buried by eroded sediments from uplands and by precipitation of gypsum from rising groundwater. The sedimentation occurred largely between the Preclassic and Late Classic, when ancient Maya farmers built canals in pre-existing low spots to reclaim these wetlands. Thus, stable natural processes, environmental change, and human manipulation have acted together to form patterned wetland ground over the later Holocene.
GC51C-1061 0800h
Late-Quaternary Glacial History of the Venezuelan Andes
Lake sediments and bog deposits in the Cordillera de Merida were used to identify and date multiple late-Pleistocene and Holocene glacial advances/retreats of varying magnitudes. Analyses include AMS radiocarbon dating, sedimentology studies, pollen and diatom assemblage investigations, magnetic susceptibility, geochemistry and stable isotope measurements (C, N, and O) on a series of cores from 7 lakes and 2 bogs. Additionally, equilibrium line altitude (ELA) reconstructions using remotely sensed imagery, high-resolution topographic data, and GIS mapping provide estimates of paleo-temperature associated with glacier advances. Results show that by 15.9 ka glaciers had retreated up-valley from their maximum LGM extent to a position that would remain ice covered until 6.2 ka. This retreat was interrupted by a glacier re-advance between 14 and 10.5 ka. Glaciers retreated after 10 ka, forming a series of recessional moraines but maintained positions down-valley of the location of the initial late-Pleistocene deglaciation. This glacial extent was maintained until after 6.2 ka when glaciers retreated further up valley and disappeared from some watersheds. Four small glacier advances occurred between 0.8 and 0.1 ka followed by rapid retreat and disappearance of ice from most watersheds.
GC51C-1062 0800h
Biogeochemical Indicators of Paleoenvironmental Changes in a Humid Region of Amazonia (Lagoa da Pata, Sao Gabriel da Cachoeira)
Analysis of four sediment core collected in Lagoa da Pata lake show significant changes in the environmental history of Amazonia during the last 40,000 years. The Lagoa da Pata Lake (0°17'S, 66°40'W) is located on the top of the Morro dos Seis Lagos, an isolated hill in the extensive forested plane surface of high Rio Negro basin. The climate in the region presents mean annual precipitation around 3000 mm without dry season. Three distinct sections are clearly identified in the LPT III, LPT IV and LPT V cores. They consist of upper and lower organic-rich layers, separated by a clastic layer which represents a short period of rapid accumulation around 18 ka BP. Analysis of the organic matter composition by d13C and d15N, total Carbon and Nitrogen, sedimentary chlorophyll concentration and organic matter petrography show three different phases related to organic matter deposition. Between, at least 46,000 and 31,000 14C yr B.P., the lake presented a high water level attested by high accumulation rates of organic carbon. Values of d13C (around -32%) associated to high C/N ratio demonstrate a lignocellulosic material contribution in this phase; the second phase between the 31000 yr 14C B.P. and 18000 14C B.P. is characterised by a lower productivity attested by low sedimentary chlorophyll and total organic carbon values. The d13C values increase with medium values around -28% that may correspond to an input of organic matter rich in C4 plants (grasses). The sedimentation rate decreases significantly. High charcoal fluxes were observed in this period. At around 18000 14C yr BP, there appears to have been a sudden input of clastic material. This is represented by a sandy facies, which exhibits lower carbon contents. From 18000 14C yr B.P. to the present time occurred an increase in lacustrine productivity marked by an increase in carbon and chlorophyll derivate accumulation rates. The C/N values dropped indicting an algae organic matter contribution. It probably corresponds to a lake level rise increasing the production of the system. Increases in charcoal deposition were observed during the upper and medium Holocene. This project is part of the PROFIX/CNPq (processo 54086/01-5) and CNPq (Brazil)-IRD(France) project "PALEOTROPICA" .
GC51C-1063 0800h
Late Holocene Lacustrine Records of Climate and Vegetation Change From Southern Patagonia, Chile
The westerly wind field is one of the most prominent atmospheric circulation features in the Southern Hemisphere, having a major impact on the climate of Chile and hydrographic conditions in the Southern Ocean. The latitudinal position and strength of the westerlies directly influences the amount and isotopic composition of precipitation that falls in southern Chile. Although instrumental records provide information on how the westerlies have varied over the recent past there is still an incomplete understanding of how the strength and latitudinal position of the southern westerlies have changed during the Holocene and how the wind field has varied at millennial to sub-decadal timescales. In this study we relate changes in the westerly winds to changes in water balance as recorded in closed-basin lakes. Sediment cores were obtained from Laguna Guanacos in Parque Nacional Torres del Paine (51$\deg$S, 72$\deg$W) during the austral summer of 2004 and sampled at 1-2cm intervals for pollen, charcoal, and stable isotope analysis. Laguna Guanacos is a small closed-basin lake situated in the core of westerly wind belt and is therefore sensitive to fluctuations in the strength and position of the westerlies. The sediment cores obtained from the lake reveal high concentrations of organic mater ($\sim$20%) and biogenic carbonate, which is rare in Chilean Patagonia. AMS radiocarbon dates on the organic and carbonate fractions indicate that the record spans the last $\sim$14,000 calendar years and modern dates from core tops suggest little influence by old or dead carbon sources. Pollen analysis on the late Holocene portion of the record reveals a significant expansion of the {\it Nothofagus} forest since $\sim$3500 cal yr BP, suggesting an overall increase in precipitation during this interval. Millennial- and centennial-scale fluctuations in bulk carbonate content and forest and steppe pollen are superimposed upon this pattern, providing a view of a highly dynamic westerly wind regime and forest-steppe ecotone response in southern Patagonia. These results will be discussed in conjunction with carbon and oxygen isotopic data derived from the biogenic carbonate phases exposed in the late Holocene portion of the record.
GC51C-1064 0800h
Sedimentary Iron Oxyhydroxides Fluxes in Lago Cardiel (Argentina): Estimating Late Holocene Wind Intensity in Southernmost Patagonia
In analogy to their marine counterparts, iron oxyhydroxides in lacustrine sediments are potential archives of environmental change. Eolian particles, commonly coated with Fe oxides, are the most frequent source of detritic iron oxyhydroxides in lake basins. They provide a climate-sediment connection that is applied here to an ultra high-resolution study of a laminated core from Southern Patagonia. These results show fluctuations in the amount of iron oxyhydroxides covering the last ca. 1800 years that are attributed to past changes in wind intensity. Located at $49\deg$S on the Patagonian plateau of Argentina, Lago Cardiel closed-basin occupies a tectonic depression of ca. 20 km diameter and a maximum water depth of 76 m. Today, this very arid but sensitive rain-shadow area west of the Andes is seasonally influenced by the southern westerlies. Thus, the lake provides an ideal location to trace changes in their strength and influence on the local sedimentological regime through time. A ca. 10 m long sedimentary core with a high sedimentation rate of approximately 56 cm/100 yr allows the detailed study of laminated sediments with variable degree of development and thickness. Magnetic susceptibility microprofiles at one mm scale reveals comparatively higher values in the black laminae than the light olive grey silty clay. Analogously, these changes are mirrored by fluctuations in trace element content that may indicate a seasonal change in the influx of iron oxyhydroxides to the basin. Image analyses in the laminated sections of the core show a conspicuous frequency of changes that can be attributed to the forcing mechanisms behind their formation. Thus, these new data with a higher temporal resolution refine previous results from the same core and relate these changes to local wind activity, which seems to be directly induced by the intensification or weakening of the westerly storm tracks. Furthermore, it provides an independent proxy for wind intensity confirming previous studies that were based on indicators at a lower time resolution such as seismic stratigraphy and pollen analysis.
GC51C-1065 0800h
High-Resolution Record of Hydrological Variability From Semiarid Southern Patagonia
A multiple-dated lacustrine record from the 100 m deep maar Laguna Potrok Aike in Santa Cruz, Argentina (52$^{o}$S, 70$^{o}$23`W) provides a continuous high-resolution (sedimentation rate $>$1 mm yr$^{-1}$) sediment record since $>$16,000 cal. BP subsampled with decadal resolution. In the semiarid desert (annual precipitation $<$200 mm) of southern Patagonia this is one of the few permanently water-filled lakes, which, due to its water depth, provides a continuous sedimentary record that has never dried out completely during periods of desiccation, a process frequently occurring in most of the numerous shallow lakes. Modern interannual lake level variations in the order of up to 7 m as well as fossil subaerial and subaquatic lake level terraces document a closed lake system exceptionally sensitive to changes in the hydrological regime. The total range of late Quaternary lake level fluctuations amounts to more than 50 m. Modern limnological observations show that precipitation of calcite in this hard water lake is a phenomenon coinciding with lake level fluctuations: falling lake levels increase the ionic concentration in the water column to an amount that calcite precipitates in larger quantities whereas higher lake levels do not support the production of autochthonous calcite. Thus, periods with calcite concentration of up to 38% alternate with periods in the sedimentary record with no calcite at all. As the concentration of total inorganic carbon in the sedimentary record serves as a proxy for the intensity of calcite precipitation, it is applied for the reconstruction of past lake level fluctuations at Laguna Potrok Aike. In the framework of the project South Argentinean Lake Sediment Archives and modeling (SALSA) it was thus possible to document rapid climatic changes as reflected in hydrological fluctuations. During the late Middle Ages (ca. AD 1230-1410) a rather low lake level is representative for the "Medieval Climate Anomaly". This period was followed by a prolonged period of considerably higher lake level related to the "Little Ice Age" (ca. AD 1480-1940). During the 20$^{th}$ century warming the lake level dropped again, however, not reaching medieval dimensions. Large changes in water balance, like during the last millennium, are documented for the record from Laguna Potrok Aike throughout the mid-Holocene, although extended periods of high lake levels can only be reported for the ending glacial period, the Late-Glacial and the early Holocene. A possible explanation for these hydrological variations are latitudinal shifts of the southern westerlies related to an increase or a decrease of the pressure gradient between the tropics and Antarctica during the late Quaternary leading to increased or reduced precipitation in the area of investigation.
http://www.salsa.uni-bremen.de
GC51C-1066 0800h
Lowland Neotropical Climate Change during the Late Deglacial and Early Holocene
The transition from arid glacial to moist early Holocene conditions represented a profound change in lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition ($\sim$11,250 to 7,500 cal yr BP) inferred from sediment cores retrieved in Lake Peten-Itza, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by $\sim$11,250 cal yr BP, but sediment properties indicate that lake level was $>$35 m below modern stage. From 11,250 to 10,350 cal yr BP, in the Preboreal period, lithologic changes in sediments from deep-water cores ($>$50 m below modern water level) indicate several wet-dry cycles that correlate with glacial meltwater pulses into the Gulf of Mexico. After 10,350 cal yr BP, multiple sediment proxies suggest a shift to persistent mesic climate during the early Holocene, with the exception of brief (100-300 yrs) returns to drier conditions at 10,200, 9,400, and 8,000 cal yr BP that were associated with cooling in the North Atlantic and Greenland. Comparison of results from Lake Peten-Itza with other records from the circum-Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period was tightly linked to climate change in the high-latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores-Bermuda high-pressure system. This mechanism operated on millennial-to-submillennial timescales and responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and deep-water production rates.