https://doi.org/10.1130/G47079.1 Manuscript received 7 October 2019 Revised manuscript received 22 January 2020 Manuscript accepted 31 March 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 18 May 2020 A sub-centennial-scale optically stimulated luminescence chronostratigraphy and late Holocene flood history from a temperate river confluence Ben Pears1, Antony G. Brown1,2, Phillip S. Toms3, Jamie Wood3, David Sanderson4 and Richard Jones5 1 Palaeoenvironmental Laboratory, Department of Geography and Environmental Science, University of Southampton, Highfield Campus, University Road, SO17 1BJ Southampton, UK 2 Natural Sciences, Tromsø University Museum, Arctic University of Norway, 9013 Tromsø, Norway 3 Luminescence Dating Laboratory, School of Natural and Social Sciences, University of Gloucestershire, Swindon Road, GL50 4HZ Cheltenham, UK 4 Scottish Universities Environmental Research Centre, Rankine Avenue, Scottish Enterprise Technology Park, G75 0QF East Kilbride, UK 5 Centre for English Local History, University of Leicester, Salisbury Road, LE1 7RH Leicester, UK ABSTRACT In upland reaches, 14C probability density River confluences can be metastable and contain valuable geological records of catchment functions can yield depositional histories from response to decadal- to millennial-scale environmental change. However, in alluvial reaches, flood channel and flood deposits (Macklin et al., stratigraphies are particularly hard to date using 14C. In this paper, we use a novel combination 2010), but many 14C-derived chronostratigra- of optically stimulated luminescence and multiproxy sedimentological analyses to provide a flood phies terminate before 1000 yr B.P. as a result record for the confluence of the Rivers Severn and Teme (United Kingdom) over the past two of sediment reworking and organic transloca- millennia, which we compare with independent European climate records. The results show that tion, leading to problems with the calibration by ca. 2000 yr B.P., the Severn-Teme confluence had stabilized and overbank alluviation had of dates. Additionally, in clastic-dominated allu- commenced. Initially, this occurred from moderately high flood magnitudes between ca. 2000 vial sequences, suitable organic material is only and 1800 yr B.P. (50 BCE–150 CE), but was followed from 1800 to 1600 yr B.P. (150–350 CE) occasionally encountered, and, where it is, its by fine alluvial deposition and decreased flood intensity. From 1600 to 1400 yr B.P. (350–550 reliability for dating is also questionable. These CE), the accumulation rate increased, with evidence of large flood events associated with the factors have made it difficult to relate the sedi- climatic deterioration of the Dark Age Cold Period. Following a period of reduced flood activity mentary record to documentary or instrumen- after ca. 1400 yr B.P. (ca. 550 CE), larger flood events and increase in accumulation rate once tal flood histories, which in Europe rarely go again became more prevalent from ca. 850 yr B.P. (ca. 1100 CE), coincident with the start of back further than 100–250 yr (Macdonald and the Medieval Climate Anomaly, a period associated with warmer, wetter conditions and in- Sangster, 2017; Longfield et al., 2018). In this creased land-use intensity. This state persisted until ca. 450 yr B.P. (ca. 1500 CE), after which paper, we show how the confluence of the Rivers increased flood magnitudes can be associated with climatic variations during the Little Ice Age. Severn and Teme (United Kingdom) has evolved We demonstrate that from the combination of high-resolution dating techniques and multiple over the past two millennia. We reconstruct analytical parameters, distinctive phases of relative flood magnitude versus flood duration can overbank sedimentation and a detailed flood be determined to a detailed chronological precision beyond that possible from 14C dating. This history using extensive optically stimulated permits the identification of the regional factors behind floodplain sedimentation, which we luminescence (OSL) dating, alongside detailed correlate with the intensification of land-use and climatic drivers over the last two millennia. stratigraphic and sedimentological analyses. INTRODUCTION et al., 2007; Brown et al., 2013; Dixon et al., CONFLUENCE METASTABILITY River floodplains contain archives of chang- 2018). Additionally, oscillatory channel behav- The Severn-Teme confluence (centered ing climatic and catchment conditions through ior can also occur, stimulated by large floods on 52.166947°N, 2.2303104°W) has >5 m of their extensive sedimentary records. In partic- (Brown et al., 2013). However, some conflu- overbank sediments deposited unconformably ular, confluence zones act as nodal points for ences can be surprisingly stable, becoming fixed on channel gravels. Both the Severn (4325 km2) flooding, because floodplains are generally at or pinned when confined by tall banks created by and Teme (1648 km2) catchments drain the their widest and integrate flood peaks from sev- high rates of levee and/or overbank deposition Cambrian Mountains, receiving mixed-intensity eral catchments. Confluence migration can hap- (Dixon et al., 2018). It is these large volumes of precipitation from west-to-east cyclonic activ- pen through channel-belt movements as well as overbank sediment stored in confluence zones ity and depression systems. The 3.5 km2 con- from meander-migration and/or through mean- that act as archives of flooding and past catch- fluence zone is bounded by Pleistocene gravel der cutoffs (Best and Lane, 2004; Camporeale ment conditions. terrace deposits with alluvium deriving from the CITATION: Pears, B., et al., 2020, A sub-centennial-scale optically stimulated luminescence chronostratigraphy and late Holocene flood history from a temperate river confluence: Geology, v. 48, p. 819–825, https://doi.org/10.1130/G47079.1 Geological Society of America | GEOLOGY | Volume 48 | Number 8 | www.gsapubs.org 819 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/8/819/5095985/819.pdf by guest on 25 September 2021 3.0°W 2.5°W 2.0°W aries, which continue to follow the present river Catchment location Shrewsbury A - Severn catchment course, as well as the location of medieval mills, N Buildwas N leats (artificial watercourses or aqueducts sup- plying water to watermills), and weirs (low head 52.5°N dams) all indicate channel stability from as early Worcester as 1188–951 yr B.P. (762–999 CE) (Fig. 1). The UK extensive depths of sandy silts blanketing the River Broadwas paleochannels can be traced across the conflu- Severn A ence and further afield, including the River Avon catchment Severn-Teme B confluence Tewkesbury catchment to the east. This unit was originally 52.0°N designated the “buff red silty clay” (Shotton 1978) and shown to postdate ca. 3600 yr B.P. B - Severn-Teme confluence and Powick environs Gloucester 2.25°W 2.235°W 2.22°W (ca. 1650 BCE) in all locations. B 52.175°N Section S1 km The stratigraphy of the sampled Powick location 0 25 50 75 section (52.169259°N, 2.2376946°W) (Fig. 2) S1 WSW Sample section ENE S N consists of a lower unit of 0.6 m of yellowish- S2 20 location on River Teme gray medium to fine sandy silt (unit A), over- ) 18 Powick Hams D b O 16 lain by 2.3 m of dark-yellow to reddish-brown A RF e d m d d d 52.166°N ( f 14 fine silt (unit B), covered by 1.1 m of reddish- S1 n o i t brown medium sandy silt (unit C), capped by S2 a 12 v and e Powick l E 0.8 m of dark reddish-brown coarse sandy silt S3 10 Powick Hams KEY: and a 0.4 m soil horizon (unit D). Faint, but Cores and transects 8 Paleochannels 0 0.2 0.4 0.6 0.8 11.2 1.4 1.6 1.8 2 clear, sand-silt laminations were identified and Prehistoric/Roman S2 Distance (km) Settlement (a-c) NW SE W E 52.157°N 25 River River recorded throughout. These could be traced the Medieval manor Teme Severn Church ) entire length of the section (150 m) and corre- Medieval bridge D 20 S4 SEVERN O Powick Hams Strip fields A b spond to a sequence previously recorded ∼500 m m Parish boundary ( 15 e S4 Water mill n o upstream (Brown, 1985). This confirms that the i Weir Dated t a 10 Fish pond/fishery Callow End 0 0.25 0.5 0.75 1 N v sequence dated and analyzed here is representa- e 1651 Bridge of boats l p paleochannel E Modern bridge S3 km 5 tive of the confluence as a whole, and that the C - Confluence floodplain transects (S1, S2, S3, S4) 0 0.4 0.8 1.21.6 2 2.4 2.6 rate of overbank deposition was high enough to Distance (km) S4 W S3 E SW NE W E Floodplain transects key: outpace rare bioturbation by earthworms. 25 Carey’s Callow End River 25 Carey’s River Topsoil ) brook paleochannel Severn ) brook Severn Made ground D 20 D 20 Dark red/brown alluvium O O Red/brown alluvium A RF A SAMPLING AND ANALYSIS m d Yellow alluvium d m ( ( 15 15 e d d dd d d d Grey alluvium n d n Eight OSL dates and in situ dosimetry were o o Silty clay paleochannel fill i i t t Organic paleochannel fill a 10 a 10 v 4 v Sand and gravel terrace taken at 0.4–0.8 m intervals up-sequence (see e ¹ C - B e l l Sidmouth mudstone E E 1 5 ¹4C - A 5 d Boundary ditch the Supplemental Material , Data Set S3). Sedi- e Flood embankment 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0 0.4 0.8 1.2 1.6 2 2.4 f Fishpond ment u-channels (method to obtain longitudinal Distance (km) Distance (km) p Paleochannel RF Ridge and furrow cultivation ¹4C - A: 4826-4407 yr B.P.