245 research outputs found
Directional Footing, Degeneracy, and Alignment
No full textThis paper argues from an Optimality Theory perspective that no one-to-one correspondence exists between directional footing effects and individual constraints. Recent work in OT claims that directional footing effects are best captured by the constraints All-Feet-Left (Ft-Left) and All-Feet-Right (Ft-Right) in (1) (e.g. McCarthy & Prince 1993b, 1994; Kirchner 1993; Cohn & McCarthy 1994; Crowhurst & Hewitt, in press; Hewitt 1994a; Kager 1994).
(1) a. All-Feet-Left: Align(Foot, L, PrWd, L)
b. All-Feet-Right: Align(Foot, R, PrWd, R)
This paper argues that the relationship between the alignment constraints in (1) and directional footing is more complicated than has been envisioned. In fact, the OT account presented here reveals directional effects to be epiphenomenal: either of the constraints in (1) may yield rightward or leftward footing, depending on its interaction with constraints requiring syllable-to-foot parsing and binary foot structure (see below). We also show that directionality and stray syllable parsing at edges are dependent: right-to-left and left-to-right effects under Ft-Left dominance co-occur with either the presence or the absence of a degenerate foot, but not with both. This relationship is inverted when Ft-Right dominates Ft-Left. One outcome of this study is that interactions among a small number of constraints leads to a modified typological view of metrical patterns familiar from earlier work.The definitive version of this paper was published in NELS 25: Proceedings of the North East Linguistics Society (1995) and is available at http://glsa.hypermart.net/Crowhurst, M., & Hewitt, M. S. (1995). Directional footing, degeneracy, and alignment. In J. N. Beckman (Ed.), NELS 25: Proceedings of the North East Linguistics Society (pp. 47-61). Amherst, MA: GLSA (Graduate Linguistic Student Association), Dept. of Linguistics, University of Massachusetts
Southwest Pacific deep-water carbonate chemistry during the Mid-Pleistocene Transition
No full textAfter more than 40 years of research, there is still wide disagreement in defining when the Mid-Pleistocene Transition (MPT) occurred, with climate reconstructions ranging from an abrupt versus gradual transition that began as early as 1500 ka and ended as late as 600 ka. Our recent work in the Southwest Pacific (Ocean Drilling Program Site 1123) has provided some evidence for a rapid transition, suggesting that the MPT was initiated by an abrupt increase in global ice volume 900 thousand years ago [1]. This study uses shallow-infaunal benthic foraminifera Uvigerina spp. to disentangle the contributions of deep-water temperature (using Mg/Ca ratios) and ice volume to the oxygen isotopic composition of foraminiferal calcite over the last 1.5 Ma. The resulting sea-level reconstruction across the MPT shows that the critical step in ice-volume variation was associated with the suppression of melting in Marine Isotope Stage (MIS) 23, followed by renewed ice growth in MIS 22 to yield a very large ice sheet with 120 m of sea level lowering.
Here, we built on this work with the aim to investigate further the abrupt event centered on MIS 24 to 22 (the ‘900-ka event’) and try to shed some light on the processes and mechanisms that caused the MPT. Different hypotheses account for the origin of the MPT as a response to long-term ocean cooling, perhaps because of lowering CO2. To better quantify the role of the carbon system during the MPT, we reconstruct past changes in bottom water inorganic carbon chemistry from the trace element (B/Ca) and stable isotopic composition of calcite shells of the infaunal benthic foraminifera Uvigerina spp. from 1100 ka to 350 ka at ODP Site 1123. This site was retrieved from Chatham Rise, east of New Zealand in the Southwest Pacific Ocean (41o47.2’S, 171o 29.9’ W, 3290 m water depth) and lies under the Deep Western Boundary Current (DWBC) that flows into the Pacific Ocean, and is responsible for most of the deep water in that ocean; DWBC strength is directly related to processes occurring around Antarctica. The ratio of boron to calcium (B/Ca) in benthic foraminifer shells has proven to be a reliable indicator of the calcite saturation state of ocean bottom waters. The comparison between benthic foraminifera δ18O and δ13C shows a similar trend at ODP Site 1123, implying a close relationship between these climate and carbon cycle signals, and we use our B/Ca record reconstructed from the same samples to explore the potential processes behind this tight coupling.
These results permit preliminary discussion on the deep-water carbonate saturation state during glacial/interglacial cycles. Deep-water temperatures estimates using Mg/Ca and oxygen isotopic composition of seawater (δ18Osw) are available from Site 1123 for the last 1.5 million years [1] and the phase relationship between the different signals is tentatively assessed for the early/middle Pleistocene, when different patterns of climate variability have been inferred from marine and ice cores records.
[1] Elderfield et al. (2012). Evolution of ocean temperature and ice volume through the Mid Pleistocene Climate Transition. Science, vol. 337, 6095, 704-70
Benthic Foraminiferal Oxygen Isotope Offsets Over The Last Glacial-Interglacial Cycle
No full textThe oxygen isotope (?18O) offset between contemporaneous benthic foraminiferal species is often assumed constant with time and geographic location. We present an inventory of benthic foraminiferal species ?18O offsets from the major ocean basins covering the last glacial-interglacial cycle, showing that of the twenty down-core records investigated, twelve show significant temporal changes in ?18O offsets that do not resemble stochastic variability. Some of the temporal changes may be related to kinetic fractionation effects causing deglacial/interglacial enrichment or glacial depletion in mainly infaunal species, but additional research is needed to confirm this. In addition to stratigraphic implications the finding of temporally varying offsets between co-existing benthic foraminiferal species could have implications for sea-level, deep water temperature, and regional deep water ?18O estimates
Cross calibration between XRF and ICP-MS for high spatial resolution analysis of ombrotrophic peat cores for palaeoclimatic studies
No full textOmbrotrophic peatlands are remarkable repositories of high-quality climatic signals because their only source of nutrients is precipitation. Although several analytical techniques are available for analysing inorganic components in peat samples, they generally provide only low-resolution data sets. Here we present a new analytical approach for producing high-resolution data on main and trace elements from ombrotrophic peat cores. Analyses were carried out on a 7-m-long peat core collected from Danta di Cadore, North-Eastern Italy (46A degrees 34' 16aEuro(3) N, 12A degrees 29' 58aEuro(3) E). Ca, Ti, Cr, Fe, Cu, Zn, Ga, Sr, Y, Cd, Ba and Pb were detected at a resolution of 2.5 mm with a non-destructive X-ray fluorescence core scanner (XRF-CS). Calibration and quantification of the XRF-CS intensities was obtained using collision reaction cell inductively coupled plasma quadruple mass spectrometry (CRC-ICP-QMS). CRC-ICP-QMS measurements were carried out on discrete samples at a resolution of 1 cm, after dissolution of 150-mg aliquots with 9 ml HNO3 and 1 ml HF at 220 A degrees C in a microwave system. We compare qualitative XRF-CS and quantitative CRC-ICP-MS data and, however the several sources of variability of the data, develop a robust statistical approach to determine the R (2) and the coefficient of a simple regression model together with confidence intervals. Perfect positive correlations were estimated for Cd, Cr, Pb, Sr, Ti and Zn; high positive correlations for Ba (0.8954), Y (0.7378), Fe (0.7349) and Cu (0.7028); while moderate positive correlations for Ga (0.5951) and Ca (0.5435). With our results, we demonstrate that XRF scanning techniques can be used, together with other well-established geochemical techniques (such as ICP-MS), to produce high-resolution (up to 2.5 mm) quantitative data from ombrotrophic peat bog cores
Eccentricity signal in the nannofossil time-series across the Mid-Pleistocene Transition in the northwestern Paci c Ocean (ODP Site 1209)
No full textThe Mid-Pleistocene Transition (MPT; 1.25–0.6 million years ago, Ma) is one of the most important and still debated climate reorganizations during which the glacial/interglacial cycles switched from a 41-thousand years (kyr) cycle (i.e. obliquity) to a quasi-periodic 100-kyr cycle (associated with orbital eccentricity). Variations in the orbital geometry can affect the abundance and distribution of certain marine biota such as the coccolitho- phores, a group of unicellular calcifying phytoplankton, whose skeletal remains – called nannofossils – represent a valid tool within the geological archives to infer change in surface water conditions and/or coccolithophore productivity and how orbital variations may have impacted them. Here, we apply for the rst time various time series analytical techniques to the nannofossil dataset from mid-latitudinal Ocean Drilling Program (ODP) Site 1209 in the northwest Paci c Ocean for the interval spanning the last 1.6 Myr. To better interpret the orbital signal recorded by different nannofossil species we used time series analyses (i.e. wavelet, autocorrelation and cross correlation) to identify the main periodicities by single nannofossil species during the MPT, and to investigate further their response timings to those orbital drivers. In addition, we investigated how the recorded periodicities can improve understanding of the paleoecological preferences of particular species. The combina- tion of multiple time series analyses allowed identi cation of the 100-kyr periodicity as the main cyclicity recorded in most analyzed species at Site 1209, documenting the predominance of the eccentricity-related signal at mid-latitudes and a reduced or absent in uence of the obliquity response. Thus, our data highlight how orbital in uence varies by latitude impacting the nannofossil species. The lag between eccentricity and species abun- dance uctuations was also investigated, identifying a fast response ranging between 20 and 40 kyr for the taxa Calcidiscus leptoporus subspecies leptoporus, Gephyrocapsa caribbeanica small, and Reticulofenestra spp. (>5 μm). This study corroborates the potential of nannofossils to deepen understanding of the dynamics and effects of variations in orbital geometry through time. It also underlines the need to extend the study of the responses of speci c species through the use of different time series analysis techniques in order to return complementary information and detect clearer orbital signals
Climate-induced variability in Mediterranean outflow to the North Atlantic Ocean during the late Pleistocene
No full textMediterranean Outflow Water (MOW) adds salt and density to open ocean intermediate waters and is therefore an important motor of Atlantic meridional overturning circulation (AMOC) and climate variability. However, the variability in strength and depth of MOW on geological timescales is poorly documented. Here we present new detailed records, with excellent age control, of MOW variability from 416 ka to present from rapidly accumulated marine sediments recovered from the West Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Our records of x-ray fluorescence (XRF), physical grain size and palaeocurrent information from the anisotropy of magnetic susceptibility (AMS) indicate (i) a close relationship between the orientation of principle AMS axes and glacial-interglacial cycles and (ii) two distinct regimes of MOW behaviour over the last ~416 kyrs in grain size and AMS variability at orbital (mainly precessional) and suborbital timescales. Between marine isotope stages (MIS) 10 and MIS 4, MOW was focused at a generally shallow depth on the West Iberian Margin, and changes in MOW strength were strongly paced by precession. A transition interval occurred during MIS 5 and 4, when MOW deepened and millennial-scale variability in strength flow strength was superimposed on orbitally paced change. During MIS 11 and from MIS 3 to present, MOW was deeply focused and millennial-scale variability dominated. We infer that late Pleistocene variability in MOW strength and depth were strongly climate- influenced and that changes in circum-Mediterranean rainfall climate were likely a primary control
The Marine Isotope Stage 19 in the mid-latitude North Atlantic Ocean: astronomical signature and intra-interglacial variability
No full textSince the seminal work by Hays et al. (1976), a plethora of studies has demonstrated a correlation between orbital variations and climatic change. However, information on how changes in orbital boundary conditions affected the frequency and amplitude of millennial-scale climate variability is still fragmentary. The Marine Isotope Stage (MIS) 19, an interglacial centred at around 785 ka, provides an opportunity to pursue this question and test the hypothesis that the long-term processes set up the boundary conditions within which the short-term processes operate. Similarly to the current interglacial, MIS 19 is characterised by a minimum of the 400-kyr eccentricity cycle, subdued amplitude of precessional changes, and small amplitude variations in insolation. Here we examine the record of climatic conditions during MIS 19 using high-resolution stable isotope records from benthic and planktonic foraminifera from a sedimentary sequence in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313) in order to assess the stability and duration of this interglacial, and evaluate the climate system's response in the millennial band to known orbitally induced insolation changes. Benthic and planktonic foraminiferal delta O-18 values indicate relatively stable conditions during the peak warmth of MIS 19, but sea-surface and deep-water reconstructions start diverging during the transition towards the glacial MIS 18, when large, cold excursions disrupt the surface waters whereas low amplitude millennial scale fluctuations persist in the deep waters as recorded by the oxygen isotope signal. The glacial inception occurred at similar to 779 ka, in agreement with an increased abundance of tetra-unsaturated alkenones, reflecting the influence of icebergs and associated meltwater pulses and high-latitude waters at the study site. After having combined the new results with previous data from the same site, and using a variety of time series analysis techniques, we evaluate the evolution of millennial climate variability in response to changing orbital boundary conditions during the Early-Middle Pleistocene. Suborbital variability in both surface- and deep-water records is mainly concentrated at a period of -11 kyr and, additionally, at similar to 5.8 and similar to 3.9 kyr in the deep ocean; these periods are equal to harmonics of precession band oscillations. The fact that the response at the similar to 11 kyr period increased over the same interval during which the amplitude of the response to the precessional cycle increased supports the notion that most of the variance in the 11 kyr band in the sedimentary record is nonlinearly transferred from precession band oscillations. Considering that these periodicities are important features in the equatorial and intertropical insolation, these observations are in line with the view that the low-latitude regions play an important role in the response of the climate system to the astronomical forcing. We conclude that the effect of the orbitally induced insolation is of fundamental importance in regulating the timing and amplitude of millennial scale climate variability. (C) 2014 Elsevier Ltd. All rights reserved
Response of Iberian Margin sediments to orbital and suborbital forcing over the past 420 ka
No full textHere we report 420 kyr long records of sediment geochemical and color variations from the southwestern Iberian Margin. We synchronized the Iberian Margin sediment record to Antarctic ice cores and speleothem records on millennial time scales and investigated the phase responses relative to orbital forcing of multiple proxy records available from these cores. Iberian Margin sediments contain strong precession power. Sediment “redness” (a* and 570–560 nm) and the ratio of long-chain alcohols to n-alkanes (C26OH/(C26OH + C29)) are highly coherent and in-phase with precession. Redder layers and more oxidizing conditions (low alcohol ratio) occur near precession minima (summer insolation maxima). We suggest these proxies respond rapidly to low-latitude insolation forcing by wind-driven processes (e.g., dust transport, upwelling, precipitation). Most Iberian Margin sediment parameters lag obliquity maxima by 7–8 ka, indicating a consistent linear response to insolation forcing at obliquity frequencies driven mainly by high-latitude processes. Although the lengths of the time series are short (420 ka) for detecting 100 kyr eccentricity cycles, the phase relationships support those obtained by Shackleton []. Antarctic temperature and the Iberian Margin alcohol ratios (C26OH/(C26OH + C29)) lead eccentricity maxima by 6 kyr, with lower ratios (increased oxygenation) occurring at eccentricity maxima. CO2, CH4, and Iberian SST are nearly in phase with eccentricity, and minimum ice volume (as inferred from Pacific δ18Oseawater) lags eccentricity maxima by 10 kyr. The phase relationships derived in this study continue to support a potential role of the Earth's carbon cycle in contributing to the 100 kyr cycle
Dynamics controlling the seasonal cycle of Congo Basin evaporation
No full textEvaporation is an integral component of Congo Basin climate and a crucial moisture source for basin rainfall. However, the dynamics controlling the climatological seasonal cycle of basin evaporation are unclear. Therefore, this thesis aims to make progress towards a complete understanding of the dynamics controlling the seasonal cycle of Congo Basin evaporation. Knowledge of these dynamics will provide a theoretical framework useful for advancing basin hydrological science, and a baseline against which global climate models can be evaluated.
The LandFlux-EVAL evaporation benchmark product is used in this thesis as a point of reference for Congo Basin evaporation. On the basin-wide average, LandFlux-EVAL evaporation increases from January to its one peak in March, is stable between March and April, decreases from April to a trough in July, and recovers between July and January. Evaporation is also lower at the November rainfall peak than the March rainfall peak, despite similar rainfall in the CHIRPS2 rainfall product. Using the ERA5-Land reanalysis, which effectively reproduces the pattern of lower evaporation in November than March in LandFlux-EVAL, this thesis derives plausible dynamics responsible for this feature of the seasonal cycle. Lower surface downward shortwave radiation (DSR) is a plausible driver of lower leaf area index (LAI) in the rainforest and northeastern savannah, which is the main reason for lower transpiration, and therefore lower evaporation, in November.
As evaporation differs substantially between the basin’s evergreen rainforest and deciduous savannah, to expand beyond the difference between the peaks a regional approach is used based on land cover type. Collectively, the dynamics that explain why evaporation is suppressed at the November rainfall peak relative to the March rainfall peak, and the dynamics controlling the seasonal cycles of evaporation in the rainforest and savannah, will explain what controls the seasonal cycle of evaporation on the basin-wide average. Therefore, this thesis seeks to derive plausible dynamics controlling the seasonal cycle of evaporation in these two ecoregions.
However, the thesis finds that ERA5-Land effectively reproduces the seasonal cycle of evaporation from LandFlux-EVAL in the savannah only. Therefore, the following plausible dynamics are derived from ERA5-Land controlling the savannah’s seasonal cycle of evaporation. These are considered from November to November to account for evaporation in one wet and one dry season. Between November and March, an increase in surface DSR explains an increase in LAI, transpiration and evaporation. Between March and April, the effects of a decrease in rainfall and increase in surface DSR offset one another, leading to stable evaporation. Between April and July, a decrease in rainfall explains why evaporation falls. Between July and November, early green-up and an increase in rainfall explain why evaporation recovers.
The lack of agreement between ERA5-Land and LandFlux-EVAL in the basin’s rainforest precludes the use of ERA5-Land to derive the dynamics controlling the seasonal cycle of evaporation here. However, this thesis suggests that using a sub-grid rainfall scheme in the land surface model of the next version of ERA5-Land could bring the seasonal cycle of evaporation from ERA5-Land into good agreement with LandFlux-EVAL in the rainforest and allow the next version of this reanalysis to be used to derive the dynamics controlling the seasonal cycle of evaporation here. This could then achieve a complete understanding on the basin-wide average of why the seasonal cycle of Congo Basin evaporation behaves as it does
Conflicting (U-Th)/He and fission track ages in apatite: Enhanced He retention not anomalous annealing behaviour
No full textCopyright © 2006 Elsevier B.V. All rights reserved.While initial studies showed a high degree of consistency between AFTA and apatite (U-Th)/He dating, an increasing number of studies are reporting apatite (U-Th)/He ages which are older than expected on the basis of apatite fission track data from the same region. We present data from a range of geological settings to document a systematic discrepancy between the two systems, which becomes more pronounced in samples with older fission track ages (except for samples with very low uranium contents). Results from a granite pebble and enclosing volcanogenic sandstone provide a well-controlled test case in which independent constraints are available on the underlying thermal history. These demonstrate that the progressive discrepancy between the two techniques arises not from anomalous fission track annealing behaviour, as has been suggested, but as a result of a change in the He retention properties of apatite. This change appears to be linked to the degree of accumulated radiation damage within the crystal lattice, although underlying mechanisms remain unclear. We suggest detailed experiments should be performed to delineate and quantify the processes responsible. Until this is achieved, we suggest that apatite (U-Th)/He studies should also incorporate apatite fission track data, as well as other low temperature indicators, in order to monitor the (U-Th)/He system response and to guard against the anomalous behaviour described here. © 2006 Elsevier B.V. All rights reserved.Paul F. Green, Peter V. Crowhurst, Ian R. Duddy, Peter Japsen and Simon P. Holfordhttp://www.elsevier.com/wps/find/journaldescription.cws_home/503328/description#descriptio
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