Dr Alex Thomas
James Martin Post-Doctoral Fellow
| Email: | alext@earth.ox.ac.uk |
| TEL: | +44[0] 1865 2(72047) Office: 72047, Plasma 2 Lab: 82143, Clean Lab (MFL4): 72013, (In order of frequency of use) |
| FAX: | +44[0] 1865 272072 |
| Homepage: | http://www.earth.ox.ac.uk/~alext |
Research Profile
Sea Level Change
Establishing rates and timings of Quaternary sea level change is one of my principle research interests. Understanding how much sea level changed in the past and when changes occurred sheds light on the potential drivers and dynamics of the Earth’s climate system.
IODP Exp 310 “Tahiti Sea Level”
I was a science party member for IODP expedition 310 “Tahiti Sea Level”. My main focus of research here was to use U-Th chronology of corals to establish the sea level history for the island. Corals offer a valuable tool for studying past sea level. Corals can be assigned to relatively narrow environmental habitats, particularly with respect to water depth. Fossil corals, therefore, can provide estimates of the paleo-elevation of sea level. Another advantage of using corals as sea level markers is that their age can be readily determined by U-Th, or radiocarbon dating.
The scientific rational for sampling these submerged reefs is to better constrain the timing and magnitudes of sea level changes, particularly from periods when sea level was lower than at present. The first major result to emerge from this expedition was the establishment of the timing of the penultimate deglaciation, showing that the phasing between a potential insolation forcing and deglaciation was not the same for all deglaciations. Using constraints from older corals the subsidence rate of the island has been constrained, which is important for the accurate reconstruction of the elevations of more recent samples and hence sea level records.
IODP Exp 325 “Great Barrier Reef Environmental Changes”
I was a science party member for both the preliminary site investigation, and the IODP drilling of the Great Barrier Reef. As with Tahiti my research is focused on establishing the chronology of sea level changes. In the case of the barrier Reef the IODP drilling focused on recovering material from early in the deglaciation when sea level was at it’s lowest.
iGlass
iGlass is a UK consortium of scientists using previous interglacials as a “looking glass” to constrain possible scenarios for future sea level change.
Improving geochronology
In addition to improving the accuracy of U-Th chronology, through improvement of analytical precision, there are a number of areas where the limit on accuracy lies in the geological imperfections of the samples rather than the skill of the analyst.
Pedogenic crusts
Establishing the timing of landscape development is important to a variety of fields (geomorphology, neotectonics, terrestrial paleoclimate, geohazards). Pedogenic carbonates, specifically pendent cements on the undersides of pebbles within of beneath a soil, offer the potential for providing such chronology. For a carbonate crust to form and be preserved the pebbles within the soil must be stabilized, hence the potential for dating landscape development. The age of carbonate precipitation can potentially be determined by U-Th dating. The main challenges to accurate age determination are: reduction of sample size requirements, and of analytical blanks; the correction of U-Th data for contaminants incorporated into the crust during precipitation.
230Th flux normalisation
Age models for marine cores are often built around correlation of variability of some parameter measured down core with another independently dated record. This could be to an orbital chronology, or to another well-dated climate archive such as an ice core or a U‑Th dated speleothem. Determining ages between tie points relies on some assumption about the variability of the sedimentation rate. The concentration of 230Th in the sediment can provide some addition constraint in this regard. In the open ocean the production of 230Th (from uranium dissolved in the water) is spatially uniform, and the high affinity of 230Th for particles means the removal to sediments is dependant only on water depth. The concentration of 230Th can therefore be though of as a constant flux proxy, enabling variability of sedimentation rate to be accounted for between tie points.
Geochemical proxies for oceanography
Pa/Th proxy
Like 230Th, 231Pa is produced in the water column from the decay of uranium, and is then removed onto sinking particles. Differences in the rates of removal allow fractionation of the between the two isotopes and advection, of the typically more soluble, Pa. The ratio of 231Pa to 230Th preserved into sediments therefore has the potential to inform us about past ocean circulation and particle scavenging behavior. Challenges to the robust interpretation of paleo data include: understanding the nature of the fractionation of 231Pa and 231Th both in the water column and into the sediments; the stability of proxy signature in the sediment and the potential for sedimentary Pa and Th to influence the water column radio-chemistry; and improving the spatial and temporal coverage of the global dataset.
Determining the distribution, and controls on fluxes, of trace elements in the ocean is fundamentally important for us to fully understand the role chemistry and physics play in the biological activity in the ocean. My research within the UK-GEOTRACES program is focused on using U-series nuclides to provide some constraints of fluxes to, from and within the ocean.
Teaching Profile
I am a college lecturer in earth sciences at Hertford College.
Selected Publications (Full Publications)
Thomas, A. L. et al. (2012). Assessing subsidence rates and paleo water-depths for Tahiti reefs using U Th chronology of altered corals. Marine Geology, 295-298, 86-94.
Negre, C., et al. (2010). Reversed flow of Atlantic deep water during the Last Glacial Maximum. NATURE, 468(7320), 84-+. doi:10.1038/nature09508
Thomas, A. L. et al. (2009). Penultimate deglacial sea-level timing from uranium/thorium dating of Tahitian corals.. Science, 324(5931), 1186-1189. doi:10.1126/science.1168754