[Show abstract][Hide abstract] ABSTRACT: The mechanisms of Late Pleistocene megafauna extinctions remain fiercely contested, with human impact or climate change cited as principal drivers. We compared ancient DNA and radiocarbon data from 31 detailed time series of regional megafaunal extinctions and replacements over the past 56,000 years with standard and new combined records of Northern Hemisphere climate in the Late Pleistocene. Unexpectedly, rapid climate changes associated with interstadial warming events are strongly associated with the regional replacement or extinction of major genetic clades or species of megafauna. The presence of many cryptic biotic transitions before the Pleistocene/Holocene boundary revealed by ancient DNA confirms the importance of climate change in megafaunal population extinctions and suggests that metapopulation structures necessary to survive such repeated and rapid climatic shifts were susceptible to human impacts.
[Show abstract][Hide abstract] ABSTRACT: X-ray fluorescence (XRF) scanning of sediment cores allows the rapid acquisition of bulk geochemical data at high resolution. The XRF core scanner data, which are expressed as elemental counts or peak areas, are mainly related to elemental concentrations, but they are also influenced by a series of sediment physical properties that vary with depth, such as bulk density, water content, organic matter content, and grain size. Here, we investigate the influence of grain size on elemental XRF peak areas by comparing ITRAX XRF core scanner measurements to ICP-AES elemental concentrations for two sediment cores with variable grain size. Results provide evidence for a limited influence of sediment grain size on XRF peak areas. This influence is negligible for sediment cores with grain-size variations of 10 μm or less. Our data also demonstrate that for cores with large grain-size variations, correcting the peak areas for water content improves the precision of the XRF measurements by a factor of three. This study therefore demonstrates that, for most sediment cores, the precision of data obtained by XRF core scanning is not significantly altered by grain-size variations.
Micro-XRF Studies of Sediment Cores, Edited by I. W. Croudace, R. G. Rothwell, 07/2015: chapter 19: pages 473-490; Springer.
[Show abstract][Hide abstract] ABSTRACT: The oceans absorb anthropogenic CO2 from the atmosphere, lowering surface ocean pH, a concern for calcifying marine organisms. The impact of ocean acidification is challenging to predict as each species appears to respond differently and because our knowledge of natural changes to ocean pH is limited in both time and space. Here we reconstruct 222 years of biennial seawater pH variability in the Sargasso Sea from a brain coral, Diploria labyrinthiformis. Using hydrographic data from the Bermuda Atlantic Time-series Study and the coral-derived pH record, we are able to differentiate pH changes due to surface temperature versus those from ocean circulation and biogeochemical changes. We find that ocean pH does not simply reflect atmospheric CO2 trends but rather that circulation/biogeochemical changes account for >90% of pH variability in the Sargasso Sea and more variability in the last century than would be predicted from anthropogenic uptake of CO2 alone.
Geophysical Research Letters 05/2015; DOI:10.1002/2015GL064431 · 4.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During the African Humid Period about 14,800 to 5,500 years ago, changes in incoming solar radiation during Northern Hemisphere summers led to the large-scale expansion and subsequent collapse of the African monsoon. Hydrologic reconstructions from arid North Africa show an abrupt onset and termination of the African Humid Period. These abrupt transitions have been invoked in arguments that the African monsoon responds rapidly to gradual forcing as a result of nonlinear land surface feedbacks. Here we present a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years using the hydrogen isotope composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana. We show that over much of tropical and subtropical Africa the monsoon responded synchronously and predictably to glacial reorganizations of overturning circulation in the Atlantic Ocean, but the response to the relatively weaker radiative forcing during the African Humid Period was more spatially and temporally complex. A synthesis of hydrologic reconstructions from across Africa shows that the termination of the African Humid Period was locally abrupt, but occurred progressively later at lower latitudes. We propose that this time-transgressive termination of the African Humid Period reflects declining rainfall intensity induced directly by decreasing summer insolation as well as the gradual southward migration of the tropical rainbelt that occurred during this interval.
[Show abstract][Hide abstract] ABSTRACT: A record of the hydrogen isotopic composition of terrestrial leaf waxes ( ) in sediment cores from Lake Titicaca provides new insight into the precipitation history of the Central Andes and controls of South American Summer Monsoon (SASM) variability since the last glacial period. Comparison of the record with a 19-kyr record from the nearby Illimani ice core supports the interpretation that precipitation is the primary control on with a lesser but significant role for local evapotranspiration and other secondary influences on . The Titicaca record confirms overall wetter conditions in the Central Andes during the last glacial period relative to a drier Holocene. During the last deglaciation, abrupt shifts correspond to millennial-scale events observed in the high-latitude North Atlantic, with dry conditions corresponding to the Bølling-Allerød and early Holocene periods and wetter conditions during late glacial and Younger Dryas intervals. We observe a trend of increasing monsoonal precipitation from the early to the late Holocene, consistent with summer insolation forcing of the SASM, but similar hydrologic variability on precessional timescales is not apparent during the last glacial period. Overall, this study demonstrates the relative importance of high-latitude versus tropical forcing as a dominant control on glacial SASM precipitation variability.
[Show abstract][Hide abstract] ABSTRACT: Global climate change and anthropogenic activities are threatening the future survival
of coral reef ecosystems. The ability of reef-building zooxanthellate coral to survive these
stressors may be determined through fundamental differences within their symbiotic dinoflagellates
(Symbiodinium sp.). We define the in vitro apoptotic response of 2 evolutionarily distant Symbiodinium
sp., subtypes B2 and C1, to determine the synergistic effects of disease and temperature
on cell viability using flow cytometry. The putative yellow band disease (YBD) consortium of
Vibrio spp. bacteria and temperature (33°C) had a positive synergistic effect on C1 apoptosis,
while B2 displayed increased apoptosis to elevated temperature (29 and 33°C), the Vibrio consortium,
and a lone virulent strain of V. alginolyticus, but no synergistic effects. Additionally, heat
shock protein 60 expression revealed differential cell-mediated temperature sensitivity between
subtypes via western blotting. This result marks the first evidence of Symbiodinium sp. apoptotic
variations to YBD pathogens and emphasizes the potential impact of synergistic stress on globally
distributed coral−Symbiodinium symbioses.
[Show abstract][Hide abstract] ABSTRACT: The climate of Chilean Patagonia is strongly influenced by the southern westerlies, which control the amount and latitudinal distribution of precipitation in the southern Andes. In austral summer, the Southern Westerly Wind Belt (SWWB) is restricted to the high latitudes. It expands northward in winter, which results in a strong precipitation seasonality between ∼35 and 45°S. Here, we present a new precipitation seasonality proxy record from Quitralco fjord (46°S), where relatively small latitudinal shifts of the SWWB result in large changes in precipitation seasonality. Our 1400 yr record is based on sedimentological and geochemical data obtained on a sediment core collected in front of a small river that drains the Patagonian Andes, which makes this site particularly sensitive to changes in river discharge. Our results indicate Fe/Al and Ti/Al values that are low between 600 and 1200 CE, increasing at 1200-1500 CE, and high between 1500 and 1950 CE. Increasing Fe/Al and Ti/Al values reflect a decrease in mean sediment grain-size from 30 to 20 μm, which is interpreted as a decrease in seasonal floods resulting from an equatorward shift of the SWWB. Our results suggest that, compared to present-day conditions, the SWWB was located in a more poleward position before 1200 CE. It gradually shifted towards the equator in 1200-1500 CE, where it remained in a sustained position until 1950 CE. This pattern is consistent with most precipitation records from central and southern Chile. The comparison of our record with published regional sea surface temperature (SST) reconstructions for the late Holocene shows that equatorward shifts of the SWWB are systematically coeval with decreasing SSTs and vice versa, which resembles fluctuations over glacial-interglacial timescales. We argue that the synchronicity between SST and SWWB changes during the last 1400 years represents the response of the SWWB to temperature changes in the Southern Hemisphere.
[Show abstract][Hide abstract] ABSTRACT: As sea surface temperatures rise and the global human population increases, large-scale field observations of marine organism health and water quality are increasingly necessary. We investigated the health of corals from the family Fungiidae using visual observations in relation to water quality and microbial biogeochemistry parameters along 1300 km of the Red Sea coast of Saudi Arabia. At large scales, incidence of lesions caused by unidentified etiology showed consistent signs, increasing significantly from the northern to southern coast and positively correlated to annual mean seawater temperatures. Lesion abundance also increased to a maximum of 96% near the populous city of Jeddah. The presence of lesioned corals in the region surrounding Jeddah was strongly correlated with elevated concentrations of ammonium and changes in microbial communities that are linked to decreased water quality. This study suggests that both high seawater temperatures and nutrient pollution may play an indirect role in the formation of lesions on corals.
Marine Environmental Research 07/2014; 98. DOI:10.1016/j.marenvres.2014.04.002 · 2.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Paired strontium-to-calcium (Sr/Ca) and δ18O measurements for two Porites lutea corals recovered from Hon Tre Island, Vietnam, are strongly correlated to sea surface temperature (SST) and precipitation at monthly to interannual time-scales. Least squares linear regression of monthly Sr/Ca to SST shows a strong, significant correlation (r2 = 0.77, p < .0001), with root mean square residuals of 0.9 °C. 3-year averaged (binned) Sr/Ca for wet (Sep–Nov) and dry (Jan–Mar) seasons separately captures SST variability at interannual time scales (Sr/Ca RMSR = 0.42 °C and 0.70 °C for wet and dry seasons, respectively). Coral δ18O correlates weakly to SST at seasonal and interannual time scales for wet and dry seasons, with significant anomalies (δ18O RMSR = 2.4 °C and 1.65 °C, respectively). Correcting the SST influence on coral δ18O using paired Sr/Ca values provides estimates of δ18O of seawater (δ18Osw). 3-year averaged δ18Osw during the wet season shows a significant correlation to local precipitation (r2 = 0.54, p = 0.01). These results show that coral Sr/Ca in this location accurately reflects SST at a number of timescales, and that seawater δ18O composition in the wet season is controlled by local precipitation, largely unmodified by ocean circulation during the winter monsoon. This study highlights the sensitivity and utility of coral geochemistry in this region for reliably reconstructing SST and monsoonal precipitation.
[Show abstract][Hide abstract] ABSTRACT: Lipid content and fatty acid profiles of corals and their dinoflagellate endosymbionts are known to vary in response to high-temperature stress. To better understand the heat-stress response in these symbionts, we investigated cultures of Symbiodinium goreauii type C1 and Symbiodinium sp. clade subtype D1 grown under a range of temperatures and durations. The predominant lipids produced by Symbiodinium are palmitic (C16) and stearic (C18) saturated fatty acids and their unsaturated analogs, the polyunsaturated fatty acid docosahexaenoic acid (C22:6, n-3; DHA), and a variety of sterols. Prolonged exposure to high temperature causes the relative amount of unsaturated acids within the C18 fatty acids in Symbiodinium tissue to decrease. Thermal stress also causes a decrease in abundance of fatty acids relative to sterols, as well as the more specific ratio of DHA to an algal 4-methyl sterol. These shifts in fatty acid unsaturation and fatty acid-to-sterol ratios are common to both types C1 and D1, but the apparent thermal threshold of lipid changes is lower for type C1. This work indicates that ratios among free fatty acids and sterols in Symbiodinium can be used as sensitive indicators of thermal stress. If the Symbiodinium lipid stress response is unchanged in hospite, the algal heat-stress biomarkers we have identified could be measured to detect thermal stress within the coral holobiont. These results provide new insights into the potential role of lipids in the overall Symbiodinium thermal stress response.