Simon Crowhurst - Academia.edu (original) (raw)

Papers by Simon Crowhurst

Geochimica et Cosmochimica Acta

Abstract The accuracy of the magnesium/calcium palaeotemperature proxy has been questioned, in pa... more Abstract The accuracy of the magnesium/calcium palaeotemperature proxy has been questioned, in particular because the ratio of magnesium to calcium in foraminiferal tests could be affected by local or global changes in carbonate ion concentrations in deep water. A related question regarding the technique is its problematic phase relationship to orbital eccentricity: Mg/Ca records of intermediate and deep waters typically show a phase lead with respect to orbital eccentricity. This calls into question either the validity of the Mg/Ca palaeotemperature proxy, or the assumption that orbital eccentricity is pacing the 100 kyr climate oscillations, or both. This paper addresses these questions, and suggests that a phase lead of the type observed at ODP 1123 is unlikely to be generated by the operation of the carbonate ion effect, and might be attributable to heat storage in the oceans during low eccentricity episodes.

Journal of Intellectual Disability Research, 2008

European Journal of Personality, 1999

Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, t... more Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.

Scientific Drilling, 2013

Nick Shackleton's research on piston cores from the Iberian margin highlighted the importance of ... more Nick Shackleton's research on piston cores from the Iberian margin highlighted the importance of this region for providing high-fidelity records of millennial-scale climate variability, and for correlating climate events from the marine environment to polar ice cores and European terrestrial sequences. During the Integrated Ocean Drilling Program (IODP) Expedition 339, we sought to extend the Iberian margin sediment record by drilling with the D/V JOIDES Resolution. Five holes were cored at Site U1385 using the advanced piston corer (APC) system to a maximum depth of ∼ 155.9 m below sea floor (m b.s.f.). Immediately after the expedition, cores from all holes were analyzed by core scanning X-ray fluorescence (XRF) at 1 cm spatial resolution. Ca/Ti data were used to accurately correlate from hole-to-hole and construct a composite spliced section, containing no gaps or disturbed intervals to 166.5 m composite depth (mcd). A low-resolution (20 cm sample spacing) oxygen isotope record confirms that Site U1385 contains a continuous record of hemipelagic sedimentation from the Holocene to 1.43 Ma (Marine Isotope Stage 46). The sediment profile at Site U1385 extends across the middle Pleistocene transition (MPT) with sedimentation rates averaging ∼ 10 cm kyr −1. Strong precession cycles in colour and elemental XRF signals provide a powerful tool for developing an orbitally tuned reference timescale. Site U1385 is likely to become an important type section for marine-ice-terrestrial core correlations and the study of orbital-and millennial-scale climate variability.

The isotopic composition of carbon, delta13C, in seawater is used in reconstructions of ocean cir... more The isotopic composition of carbon, delta13C, in seawater is used in reconstructions of ocean circulation, marine productivity, air-sea gas exchange, and biosphere carbon storage. We present a synthesis of delta13C measurements taken from foraminifera in marine sediment cores over the last 150,000 years, comprising previously published and unpublished data from benthic and planktonic records throughout the global ocean. Data are

One of the most intriguing aspects of Pleistocene climate is the development of quasi-periodic (c... more One of the most intriguing aspects of Pleistocene climate is the development of quasi-periodic (ca.100 kyr), high-amplitude glacial variability during the middle Pleistocene. A number of causes have been suggested for this Mid-Pleistocene Transition (MPT), but there is no consensus yet on this matter despite more than two decades of research. Most hypotheses invoke either a response to a long-term cooling, possibly induced by decreasing atmospheric pCO2, or changes in internal ice-sheet dynamics. Marine records from the North Atlantic and tropical-ocean upwelling regions have already provided support for the first hypothesis, documenting decreases of sea-surface temperatures (SSTs) during the MPT; in contrast, no discernible shift in SSTs has been identified in the Western Pacific Warm Pool. Little is known about how deep-water temperatures evolved during this climate transition, how deep-water temperatures correlated with the surface water variability and how temperature change is linked with the records of greenhouse gases and atmospheric temperature from EPICA Dome C. Here we present records of Mg/Ca and stable isotopes measured in planktonic and benthic foraminifera over the time interval 0.35-1 Ma (Marine Isotope Stages 10-27) from a marine sediment core recovered in the mid-Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). We have analysed three foraminiferal species: Globigerina bulloides, Globorotalia inflata and Uvigerina spp. to represent the upper sea-surface, the thermocline and the deep ocean respectively. The Mg/Ca-based temperature estimates allow us to test the hypothesis of a global cooling associated with the MPT in the Southern Hemisphere and, paired with the foraminiferal d18O record, derive the d18O of the water in which the foraminifer calcified, the latter of which combines a global glacioeustatic signal with local hydrographic effects. Temperature estimates suggest that the long term average surface- and deep- water temperatures changed relatively little in the South-west Pacific through the interval analysed, with no systematic shift across the MPT. In addition, these results permit preliminary discussion of the magnitude of surface- and deep-water temperature changes during glacial/interglacial transitions and the interglacials themselves. The phase relationship between surface- and deep-water signals has been assessed; in particular, benthic Mg/Ca and planktonic Mg/Ca are almost in phase at eccentricity, tilt and precession, with a slight lead of the planktonics over the benthics. At the 100 kyr period, Mg/Ca leads d18O both in the planktonic and benthic records. Finally, the phasing of variations in the marine record will be considered with respect to other component of the climate system, and a comparison with greenhouse gases and atmospheric temperatures from EPICA Dome C will be attempted, after evaluation of methods for precise synchronization of these records.

Earth and Planetary Science Letters, 1997

A broad correspondence between long pollen sequences and the deep-sea oxygen isotope record has b... more A broad correspondence between long pollen sequences and the deep-sea oxygen isotope record has been noted for some time, but there has been little effort to explore just how similar the two types of evidence are in terms of their overall structure on glacial-interglacial timescales and also how they may differ. These questions have profound importance both for how we view the stratigraphic record of changing climate in different regions and for our understanding of the climate system. Here we link the four longest European pollen records and derive a terrestrial sequence of vegetation events and a coherent stratigraphic scheme for the last 500,000 years. Comparison of the terrestrial and marine records shows good agreement, but it also reveals that the pollen sequences contain a higher degree of climate sensitivity than the oxygen isotope record. In addition, it suggests that neither an oxygen isotope record nor a Milankovitch-forced ice volume model may provide an appropriate template for fine-tuning the terrestrial record and that better chronologies will depend on an improved understanding of controls on sedimentation rates in individual sedimentary basins.

Quaternary Science Reviews, 2001

The palynological signature of interglacial deposits in the fragmentary European terrestrial reco... more The palynological signature of interglacial deposits in the fragmentary European terrestrial record has often been used as the basis for determining their chronostratigraphical position and ultimately their age. This has placed emphasis on the presence/absence and abundance of certain characteristic taxa, but given the lack of continuous stratigraphies and independent chronologies, it has been difficult to assess the extent to which this strategy has produced reliable schemes. Here, an alternative approach is adopted whereby a chronological framework is developed for long and continuous pollen sequences from southern Europe. This in turn allows the emergence of a complete stratigraphical scheme of major vegetation events for the last 430 thousand years (ka) and the evaluation of the stage record of different taxa and their potential diagnostic value for biostratigraphical correlation. The comparison shows distinct similarities among some temperate stages of the terrestrial equivalent complexes of Marine Isotope Stages (MIS) 5 and 7 and also of MIS 9 and 11, but examination of combined records of taxa provides a possibility to differentiate between individual stages. A numerically-derived dichotomous key for the terrestrial stages based on the palynological records of 10 taxa is presented. Carpinus, Fagus, Abies, Pterocarya and Buxus emerge as the best ‘indicator pollen types’ because of their variable behaviour from one stage to the next, possibly a result of their late expansion within a temperate stage or reduced genetic variability. The analysis shows that the palynological signature of a temperate deposit can constrain the range of chronostratigraphical possibilities, but vegetation and palynological variability arising from local factors could result in difficulties in making a definite assignment at individual sites.

Journal of Biogeography, 2002

Analytical and bioanalytical chemistry, 2015

Ombrotrophic peatlands are remarkable repositories of high-quality climatic signals because their... more Ombrotrophic 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 (46° 34' 16″ N, 12° 29' 58″ 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 diss...

Quaternary Science Reviews, 2015

Quaternary International, 2013

Agu Fall Meeting Abstracts, Dec 1, 2002

It is difficult to imagine a situation in which (a) a geological record built up at a constant ra... more It is difficult to imagine a situation in which (a) a geological record built up at a constant rate or (b) a geological deposit built up at a rate that was independent of climate. In addition all geological proxies for climate parameters are subject to both observational noise, and non- stationarity in their relationship with climate. Hence considerable judgement is needed in first selecting geological records that will be amenable to "tuning" to a record of the changing distribution of solar insolation as calculated by an astronomer, and then in carrying out the operation. Tuning a geological record to an incorrect insolation record, or to the wrong time interval in a correct insolation record, or to a simplified "caricature" of the true insolation record can all provide considerable insights in some circumstances. Nevertheless from the point of view of the geological time scale it is important to carry out the procedure correctly, with the best available astronomical data. It is also important that one can use the tuning to verify the astronomical calculations. We have recently re-tuned a number of records on the basis of the new calculations discussed by Laskar (2001). Astronomical Solutions for Paleoclimates Studies. Eos Trans. AGU, 82(47), Fall Meet. Suppl. Abstract U11A-01. In the Pliocene re-tuning is generally straightforward and uninformative because the astronomical changes are small and the tuning is so unambiguous that little judgement is required. In the Early Miocene and Oligocene the situation is more interesting. During this time available data suggests that the climatic variability is dominated by 41-thousand year (ky) obliquity cycles but attaining a correct tuning requires identification of the weaker precession signal and making use of the longer-term (100-ky and 400-ky) eccentricity modulation. A surprising aspect of the tuning published by us in 1999 was that between 20 and 24 My ago there was long-term (about 1.1 My) modulation of the obliquity signal that matched that predicted; this match broke down in the earlier part of the record. Re-tuning the records for this interval required a slight "stretching" to take account of the fact that 400-ky eccentricity maxima in the Oligocene are around 100 ky older than in the previous astronomical solution. This obviously required the addition of a very small number of additional obliquity cycles; thus it was necessary to make a judgement anew regarding the interpretation of each cycle. This in turn enables us to assess the degree of robustness of the tuning. The fact that in the new tuning the amplitude modulation of the obliquity signal between 20 and 24 My ago still matches the calculated obliquity modulation is not surprising, since in this time-window the amplitude envelope of obliquity has moved back in age to about the same extent as the eccentricity signal, as a result of the new calculations. However, the fact that the re-tuned data demonstrates a 1.1 My amplitude modulation of the obliquity signal that now remains in phase with the calculated signal all the way back to 30 My ago, despite the fact that the re-tuning did not entail significant re-interpretation of the record, strongly suggests both that the new solution represents an "improvement" and that the coherence with this long-term modulation constitutes a very strong independent validation of this approach to geological time scale development.

Climate of the Past Discussions, 2009

One of the most intriguing aspects of the Pleistocene climate is the development of 100 kyr glaci... more One of the most intriguing aspects of the Pleistocene climate is the development of 100 kyr glacial variability during the middle Pleistocene. Very little is known about how deep-water temperatures evolved during this climate transition, how it correlated to the surface water variability and how it links with the records of greenhouse gases and atmospheric temperature from EPICA Dome C.

Records of benthic calcite delta18O and Mg/Ca will be presented for a site in Lower Circumpolar D... more Records of benthic calcite delta18O and Mg/Ca will be presented for a site in Lower Circumpolar Deep Water in the Pacific sector of the Southern Ocean extending from the MIS 37/38 boundary at the start of the mid Pleistocene Transition. Almost half of the flux of cold bottom water entering the major basins of the world ocean does so through