Anna F. Jones

New Zealand and UK Holocene flooding demonstrates interhemispheric climate asynchrony

The timing and controls of interhemispheric Holocene climate change have remained poorly understood, primarily because of the absence of well-dated and continuous climatic records in terrestrial environments. Here we report a new probability-based meta-analysis of 1185 14 C dates from fluvial sedimentary sequences in New Zealand and the UK, which provides a robust means of identifying centennial- and multicentennial-length episodes of Holocene river flooding. Statistical analysis shows that prior to large-scale human impact, which began at ca. 1000 cal. yr B.P., the incidence of extreme floods in New Zealand and the UK has been largely asynchronous during the Holocene. Major periods of flooding are controlled by large-scale shifts in atmospheric circulation, which alter the frequency of extreme precipitation events. Our novel synthesis demonstrates that short-term climate change, of sufficient magnitude to modify flooding regimes, was out of phase in the temperate maritime regions of the Northern and Southern Hemispheres during much of the Holocene. This supports recent evidence from both glacial and marine records that Holocene climate changes may have been antiphased between the polar regions and that this could have been related to variation in the strength of deep water formation.

Recurring flood distribution patterns related to short-term Holocene climatic variability

Millennial- and multi-centennial scale climate variability during the Holocene has been well documented, but its impact on the distribution and timing of extreme river floods has yet to be established. Here we present a meta-analysis of more than 2000 radiometrically dated flood units to reconstruct centennial-scale Holocene flood episodes in Europe and North Africa. Our data analysis shows a general increase in flood frequency after 5000 cal. yr BP consistent with a weakening in zonal circulation over the second half of the Holocene, and with an increase in winter insolation. Multi-centennial length phases of flooding in UK and central Europe correspond with periods of minimum solar irradiance, with a clear trend of increasing flood frequency over the last 1000 years. Western Mediterranean regions show synchrony of flood episodes associated with negative phases of the North Atlantic Oscillation that are out-of-phase with those evident within the eastern Mediterranean. This long-term flood record reveals complex but geographically highly interconnected climate-flood relationships, and provides a new framework to understand likely future spatial changes of flood frequency.

Flood series data for the later Holocene: Available approaches, potential and limitations from UK alluvial sediments

Flood sediments characterise the alluvial record, though the series of floods these deposits record is a complex one because of variable deposition in different alluvial subenvironments. To date, flood sequences in the UK have been analysed using upland boulder berms, vertically accreting fills in contracting channels, cutoff fills and flood basin deposits. These cover different timespans ranging from c. 100—300 yr (berms and channel fills) to a limited set of the most extreme of Holocene events. Collation of future site data may allow record splicing to generate longer or more continuous series. Other sequences are potentially available from alternative depositional environments, including lateral accretion deposits, slackwater sediments in bedrock channels and palaeochannels. However, these have not produced UK results as yet, and there are reasons for them being less useful in this environment than has proved to be the case elsewhere. Finally, a new approach to overbank sedimentation sequences is presented. Analysing such deposits in much greater detail than hitherto gives flood series on a millennial timescale, and it is suggested that the prevalence of such sediments on UK floodplains may allow longer and more complete flood series data to be obtained than has otherwise been possible.

Micro-XRF Applications in Fluvial Sedimentary Environments of Britain and Ireland: Progress and Prospects

This chapter considers applications of micro-XRF scanning in fluvial depositional environments and presents case-studies from Britain and Ireland in three key river management areas: flood reconstruction; pollution and provenance mapping; and floodplain sediment dynamics. Although fluvial sediment archives are typically shorter and more fragmented than marine and lake sediment records, they do offer significant palaeoenvironmental potential, not least because of the sensitivity of river systems to environmental change. A major consideration in micro-XRF analysis, however, is the continuity and heterogeneity of alluvial sediments and the integrity of accretionary records once they have been subject to post-depositional processes, such as human disturbance and pedogenesis. Thus far, micro-XRF applications in fluvial environments have been limited. One research area currently being developed is the field of flood reconstruction, where elements and, in particular, element ratios (e.g. Zr/Rb, Zr/Ti) can be used as particle size proxies. Micro-XRF core scanning technologies allow for analysis at the event-scale, which hitherto has been unachievable in silt and clay sediments. The potential to build and significantly enhance our understanding of longer term flooding patterns and non-stationarity, offers considerable scope for augmenting instrumental records and providing new perspectives for contemporary river management. Rapid geochemical assessment of fluvial sedimentary deposits can also be used to support floodplain reconstruction studies and pollution investigations, but greater scope will emerge from the calibration of raw XRF count data to elemental concentration. In this paper we demonstrate the potential for using micro-XRF data in sediment provenance investigations, but improvements in error quantification and propagation need to be explored. Given that river alluvium plays an integral role in the cycling and storage of contaminants, further applications in this field would be hugely beneficial for river managers. Although sediment heterogeneity places significant challenges to the quantification of micro-XRF core scanner results, there has been little attempt to establish what these limitations mean in practical terms.