A sub-centennial-scale optically stimulated luminescence chronostratigraphy and late Holocene flood history from a temperate river confluence

Abstract

River confluences can be metastable and contain valuable geological records of catchment response to decadal- to millennial-scale environmental change. However, in alluvial reaches, floodstratigraphies are particularly hard to date using 14C. In this paper, we use a novel combination of optically stimulated luminescence and multiproxy sedimentological analyses to provide a flood record for the confluence of the Rivers Severn and Teme (United Kingdom) over the past two millennia, which we compare with independent European climate records. The results show that by ca. 2000 yr B.P., the Severn-Teme confluence had stabilized and overbank alluviation had commenced. Initially, this occurred from moderately high flood magnitudes between ca. 2000 and 1800 yr B.P. (50 BCE–150 CE), but was followed from 1800 to 1600 yr B.P. (150–350 CE) by fine alluvial deposition and decreased flood intensity. From 1600 to 1400 yr B.P. (350–550 CE), the accumulation rate increased, with evidence of large flood events associated with the climatic deterioration of the Dark Age Cold Period. Following a period of reduced flood activity after ca. 1400 yr B.P. (ca. 550 CE), larger flood events and increase in accumulation rate once again became more prevalent from ca. 850 yr B.P. (ca. 1100 CE), coincident with the start of the Medieval Climate Anomaly, a period associated with warmer, wetter conditions and increased land-use intensity. This state persisted until ca. 450 yr B.P. (ca. 1500 CE), after which increased flood magnitudes can be associated with climatic variations during the Little Ice Age. We demonstrate that from the combination of high-resolution dating techniques and multiple analytical parameters, distinctive phases of relative flood magnitude versus flood duration can be determined to a detailed chronological precision beyond that possible from 14C dating. This permits the identification of the regional factors behind floodplain sedimentation, which we correlate with the intensification of land-use and climatic drivers over the last two millennia.

Sedimentary ancient DNA shows terrestrial plant richness continuously increased over the Holocene in northern Fennoscandia

“…The rate of increase in taxonomic richness at the regional scale slowed during the Late Holocene, and the regional species pool clearly leveled off during the past three millennia, suggesting that an equilibrium level may have been reached. The slight cooling and well-known instability in this period ( 50 ) did not substantially affect richness. Palynological richness in northern Fennoscandia increases slightly ( 48 ) or is variable ( 24 ) during this period.…”

Sedimentary ancient DNA shows terrestrial plant richness continuously increased over the Holocene in northern Fennoscandia

“…The regional species pool clearly levelled off during the past few millennia suggesting that a near saturation point was reached. The slight cooling and well-known instability in this period (Pears et al , 2020) had no direct effect on our richness estimates or species pool, although it clearly caused a withdrawal of the forest in the region (Seppa, 1998; Sjögren & Damm, 2019). For Gauptjern, palynological richness also increases slightly in this period (Jensen & Vorren, 2008).…”

“…The regional species pool clearly levelled off during the past few millennia suggesting that a near saturation point was reached. The slight cooling and well-known instability in this period (48) negatively impacted richness, potentially as a consequence of the cooling-induced withdrawal of the forest in the region (14,21). Palynological richness in northern Fennoscandia also increases slightly (47) or is variable (21) during this period.…”

Urban flooding in Britain: an approach to comparing ancient and contemporary flood exposure

“…In the UK, medieval soil erosion under population and land pressure before the Black Death (1348-9 CE) added a blanket of fine sediment to floodplains and siltation within channels. Whether flood discharges greatly changed or not, this has led to the deposition of fine (Macklin et al, 2014;Pears et al 2020). Thus, in the Nene valley, over 2 m of alluvial clays overlie Late Saxon or medieval ridge-and-furrow (Robinson 1992), whilst at York 9 m of sediment provide a record of flood sedimentation from the second century CE (Hudson-Edwards et al 1999).…”