Douglas Peter M. J.

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Douglas
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Peter M. J.
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  • Preprint
    Pre-aged plant waxes in tropical lake sediments and their influence on the chronology of molecular paleoclimate proxy records
    ( 2014-07) Douglas, Peter M. J. ; Pagani, Mark ; Eglinton, Timothy I. ; Brenner, Mark ; Hodell, David A. ; Curtis, Jason H. ; Ma, Keith ; Breckenridge, Andy
    Sedimentary records of plant-wax hydrogen (δDwax) and carbon (δ13Cwax) stable isotopes are increasingly applied to infer past climate change. Compound-specific radiocarbon analyses, however, indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments. The influence of these time lags on interpretations of plant-wax stable isotope records is presently unconstrained, and it is unclear whether such time lags also affect lacustrine sediments. We present compound-specific radiocarbon (14Cwax) data for n-alkanoic acid plant waxes (n-C26 to n-C30) from: 1) a sediment core from Lake Chichancanab, Yucatan Peninsula, Mexico, 2) soils in the Lake Chichancanab catchment, and 3) surface sediments from three other lakes in southeastern Mexico and northern Guatemala. 14Cwax ages in the surface sediments are consistently older than modern, and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area. 14Cwax ages in soils surrounding Lake Chichancanab increase with soil depth, consistent with deep, subsoil horizons being the primary source of lacustrine aged plant waxes, which are likely delivered to lake sediments through subsurface transport. Plant waxes in the Lake Chichancanab core are 350 to 1200 years older than corresponding ages of bulk sediment deposition, determined by 14C dates on terrestrial plant macrofossils in the core. A δDwax time series is in closer agreement with other regional proxy hydroclimate records when a plant-wax 14C age model is applied, as opposed to the macrofossil-based core chronology. Inverse modeling of plant-wax age distribution parameters suggests that plant waxes in the Lake Chichancanab sediment core derive predominantly from millennial-age soil carbon pools that exhibit relatively little age variance (< 200 years). Our findings demonstrate that high-temporal-resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils. They also underscore the importance of direct radiocarbon dating of these organic molecules.
  • Preprint
    Drought, agricultural adaptation, and sociopolitical collapse in the Maya Lowlands
    ( 2014-10) Douglas, Peter M. J. ; Pagani, Mark ; Canuto, Marcello A. ; Brenner, Mark ; Hodell, David A. ; Eglinton, Timothy I. ; Curtis, Jason H.
    Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (approximately 800 to 950 CE). Evidence for drought largely derives from the drier, less populated northern Maya Lowlands, but does not explain more pronounced and earlier societaldisruption in the relatively humid southern Maya Lowlands. Here we apply hydrogen and carbon isotope compositions of plant-wax lipids in two lake sediment cores to assess changes in water availability and land use in both the northern and southern Maya lowlands. We show that relatively more intense drying occurred in the southern lowlands than in the northern lowlands during the Terminal Classic period, consistent with earlier and more persistent societal decline in the south. Our results also indicate a period of substantial drying in the southern Maya Lowlands from ~200 to 500 CE, during the Terminal Preclassic and Early Classic periods. Plant-wax carbon isotope records indicate a decline in C4 plants in both lake catchments during the Early Classic period, interpreted to reflect a shift from extensive agriculture to intensive, water-conservative maize cultivation that was motivated by a drying climate. Our results imply that agricultural adaptations developed in response to earlier droughts were initially successful, but failed under the more severe droughts of the Terminal Classic period.