Holocene Palaeoclimate
To better understand the climate system, to provide context for predictions of future rates of climate change, and to understand the role of the environment in shaping human societies a wide range of Holocene environmental information from both terrestrial and marine realms is required.
Sediments
Since my PhD, I worked on investigations of high time-resolution records of Holocene climatic change from marine sediments, mainly in high-latitude settings. My initial work on Iceland continued with Heiko Moossen during his PhD. Heiko produced multiple biomarker-based records of air-temperature, sea-surface-temperature (SST) and precipitation, from an Icelandic fjord, which revealed the leads, lags and drivers of terrestrial and marine Holocene climate in the northern North Atlantic in unprecedented detail. Work in Iceland continues with PhD student Johnathan Hall.
Since IODP Expedition 318 I have been working on Antarctic marginal sediments. Working with PhD student Kate Newton (and many international collaborators) we are exploiting a 180m Holocene sediment sequence from the Adelie Drift in the east Antarctic. The sediment cores from Site U1357 have a resolution equivalent to that of an ice core (making it a unique palaeoceanographic sub-decadal record of high latitude climate change). Our first results from this archive reveal how glacial retreat and the development of the Ross Ice Shelf triggered increased sea ice concentrations around the Antarctic coast 4,500 years ago. Water masses are cooled and freshened as they circulate underneath the ice-shelf, before feeding into the surface waters, enhancing sea-ice formation around the coast. Inclusion of these processes in global climate models may improve future projections of ocean changes around the Antarctic, in light of recent and ongoing increases in glacial melt.
I am also involved in several ongoing projects investigating environmental change recorded by lake sediments in Japan, China and S.E. Asia with many more outputs to come.
Stalagmites
Stalagmites are versatile archives of terrestrial climate, recording ambient environmental conditions at the time of deposition and preserving organic material (including biomarkers) transported from overlying soils via cave drip waters. Stalagmites ‘tap into’ the terrestrial carbon cycle in a more direct way than any other comparable paleoclimate archive (e.g., ice-cores, lakes & marine sediments). We have recently demonstrated that temperature, hydrology, vegetation and bacterial respiration signals are recorded in stalagmites by biomarkers. We did this by developing new biomarker based tools for reconstructing climate using the composition of 3-hydroxy fatty acids (3-OH-FAs – originating from bacteria). We calibrated 3-OH-FAs for reconstructing terrestrial temperature (the RAN15 proxy) and pH (the RIAN proxy) and applied these new approaches to a stalagmite to reconstruct temperature and hydrological changes over the last 9000 years. Secondly, we utilised fatty acid carbon isotopes (δ13C) extracted from a stalagmite to constrain the response of catchment vegetation and terrestrial c-cycle feedbacks during warmer phases of the Holocene finding that the catchment behaved as a carbon sink. This interpretation is uniquely applicable to stalagmites as the compound-specific records of fatty acid δ13C entrapped in calcite can be directly linked to catchment sources and processes (e.g., bacterial respiration, catchment vegetation, etc.). Such carbon isotopic interpretations are not possible from marine or lake sedimentary archives because they are produced by three end-members, plants, bacteria and algae, and subject to more geochemical and taphonomic disturbance.
Ice-Cores
The variety of climatic information available from ice-cores is probably greater than in any other natural recorder of climate, such as tree rings or sediments. Biomarker proxies have not been widely applied to glacial ice archives, despite their potential to integrate broad regional signals of biomass burning, vegetation change and other aspects of the global carbon cycle. I was part of the team to first successfully obtain information on past climate change from measurements of organic biomarkers in ice-cores, using molecules originating from plants and soils that had been transported thousands of miles, via winds, to ice-caps in Greenland and Kamchatka. We found evidence that the transport of dust from Asia to the Arctic increased (via effects on clouds and the reflective surfaces of snow and ice) the climatic influence of relatively weak changes in the strength of the sun in the past. This mechanism could also act to act to increase the effects of human-induced warming in the Arctic in the future.
James Bendle 