University of Wollongong Research Online Faculty of Science, Medicine and Health - Papers Faculty of Science, Medicine and Health 2014 Age and weathering rate of sediments in small catchments: the role of hillslope erosion Anthony Dosseto University of Wollongong, [email protected] Heather L. Buss University of Bristol Francois Chabaux University of Strasbourg Publication Details Dosseto, A., Buss, H. L. & Chabaux, F. (2014). Age and weathering rate of sediments in small catchments: the role of hillslope erosion. Geochimica et Cosmochimica Acta, 132 238-258. Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Age and weathering rate of sediments in small catchments: the role of hillslope erosion Abstract Uranium-series (U-series) isotopes in river material can be used to determine quantitative time constraints on the transfer of erosion products from source to sink. In this study, we investigate the U-series isotope composition of river-borne material in small catchments of Puerto Rico and southeastern Australia in order to improve our understanding of (i) the controls on the U-series isotope composition of river-borne material and (ii) how erosion products acquire their geochemical characteristics. In both regions, thorium isotopes track the origin of sediment and dissolved loads. Stream solutes are mainly derived from the deepest part of the weathering profile, whereas stream sediments originate from much shallower horizons, even in landslide- dominated Puerto Rican catchments. This suggests that in environments where thick weathering profiles have developed, solutes and sediments have distinct origins. The -sU eries isotope composition of stream sediments was modelled to infer a weathering age, i.e. the average time elapsed since the sediment's minerals have started weathering. In southeastern Australia, the weathering age of stream sediments ranges between 346 ± 12 kyr and 1.78 ± 0.16 Myr, similar to values inferred from weathering profiles in the same catchment. Old weathering ages likely reflect the shallow origin of sediments mobilised via near-surface soil transport, the main mechanism of erosion in this catchment. Contrastingly, in Puerto Rico weathering ages are much younger, ranging from 5.1 ± 0.1 to 19.4 ± 0.4 kyr, reflecting that sediments are derived from less weathered, deeper saprolite, mobilised by landslides. Weathering ages of stream sediments are used to infer catchment- wide, mineral-specific ew athering rates that are one to two orders of magnitude faster for Puerto Rico than for southeastern Australia. Thus, the type of erosion (near-surface soil transport vs. landslide) also affects the weathering rate of river sediments, because their weathering ages determine the potential for further weathering during sediment transport and storage in alluvial plains. Disciplines Medicine and Health Sciences | Social and Behavioral Sciences Publication Details Dosseto, A., Buss, H. L. & Chabaux, F. (2014). Age and weathering rate of sediments in small catchments: the role of hillslope erosion. Geochimica et Cosmochimica Acta, 132 238-258. This journal article is available at Research Online: http://ro.uow.edu.au/smhpapers/1696 *Manuscript 1 2 3 4 5 6 Age and weathering rate of sediments in 7 small catchments: the role of hillslope 8 erosion 9 10 11 Anthony Dosseto1,*, Heather L. Buss2 and François Chabaux3 12 13 14 15 1 Wollongong Isotope Geochronology Laboratory. School of Earth and Environmental Sciences, 16 University of Wollongong. Wollongong, NSW 2522, Australia 17 18 2 School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, UK 19 20 3 Laboratoire d’Hydrologie et de Géochimie de Strasbourg (LHyGES), Université de Strasbourg et 21 CNRS, 1 rue Blessig, 67084 Strasbourg Cedex, France 22 23 * Corresponding author: [email protected]. Tel: +61 2-4221-4805; Fax: +61 2-4221-4250 24 1 25 Abstract 26 Uranium-series (U-series) isotopes in river material can be used to determine 27 quantitative time constraints on the transfer of erosion products from source to sink. In this 28 study, we investigate the U-series isotope composition of river-borne material in small 29 catchments of Puerto Rico and southeastern Australia in order to improve our understanding of 30 (i) the controls on the U-series isotope composition of river-borne material and (ii) how erosion 31 products acquire their geochemical characteristics. In both regions, thorium isotopes track the 32 origin of sediment and dissolved loads. Stream solutes are mainly derived from the deepest 33 part of the weathering profile, whereas stream sediments originate from much shallower 34 horizons, even in landslide-dominated Puerto Rican catchments. This suggests that in 35 environments where thick weathering profiles have developed, solutes and sediments have 36 distinct origins. 37 The U-series isotope composition of stream sediments was modelled to infer a 38 weathering age, i.e. the average time elapsed since the sediment’s minerals have started 39 weathering. In southeastern Australia, the weathering age of stream sediments ranges between 40 346 ± 12 kyr and 1.78 ± 0.16 Myr, similar to values inferred from weathering profiles in the 41 same catchment. Old weathering ages likely reflect the shallow origin of sediments mobilised 42 via near-surface soil transport, the main mechanism of erosion in this catchment. Contrastingly, 43 in Puerto Rico weathering ages are much younger, ranging from 5.1 ± 0.1 to 19.4 ± 0.4 kyr, 44 reflecting that sediments are derived from less weathered, deeper saprolite, mobilised by 45 landslides. Weathering ages of stream sediments are used to infer catchment-wide, mineral- 46 specific weathering rates that are one to two orders of magnitude faster for Puerto Rico than 2 47 for southeastern Australia. Thus, the type of erosion (near-surface soil transport vs. landslide) 48 also affects the weathering rate of river sediments, because their weathering ages determine 49 the potential for further weathering during sediment transport and storage in alluvial plains. 50 3 51 Introduction 52 The measurement of uranium-series isotopes in river material can be used to determine 53 time constraints on erosion and weathering processes (Chabaux et al., 2008; Chabaux et al., 54 2006; Chabaux et al., 2003b; DePaolo et al., 2006; Dosseto et al., 2006a; Dosseto et al., 2006b; 55 Dosseto et al., 2008a; Dosseto et al., 2006c; Granet et al., 2010; Granet et al., 2007; Lee et al., 56 2010; Vigier and Bourdon, 2011; Vigier et al., 2005; Vigier et al., 2001; Vigier et al., 2006). One 57 particularly important parameter to understand the dynamics of soil formation, erosion and 58 sediment transfer at the catchment scale is the weathering age of soil and sediment as inferred 59 from U-series isotopes (termed in previous studies “residence time” or “transport time”; 60 Andersen et al., 2013; Chabaux et al., 2013; Chabaux et al., 2008; Chabaux et al., 2003a; 61 Chabaux et al., 2006; Chabaux et al., 2003b; DePaolo et al., 2006; Dequincey et al., 1999; 62 Dequincey et al., 2002; Dosseto et al., 2006a; Dosseto et al., 2006b; Dosseto et al., 2008a; 63 Dosseto et al., 2011; Dosseto et al., 2012; Dosseto et al., 2010; Dosseto et al., 2006c; Granet et 64 al., 2010; Granet et al., 2007; Keech et al., 2013; Lee et al., 2010; Ma et al., 2010; Pelt et al., 65 2008; Vigier and Bourdon, 2011; Vigier et al., 2005; Vigier et al., 2001; Vigier et al., 2006). This 66 age represents the average amount of time elapsed since the minerals that compose the soil or 67 sediment have started to chemically weather (i.e. the onset of mineral dissolution in the 68 bedrock). The weathering age of sediments integrates both their storage in weathering profiles 69 and transport in the river. Note that where the transport time in the river is short (e.g. little or 70 no storage in an alluvial plain), the weathering age of sediments gives a catchment-averaged 71 estimate of the age of weathering profiles (Fig. 1). Recent studies have shown that the 4 72 sediment weathering age can vary from as little as a few hundreds of years in Iceland (Vigier et 73 al., 2006) to several hundreds of thousands of years in the lowlands of the Amazon basin 74 (Dosseto et al., 2006a). Most of these studies have focused on large catchments: Amazon 75 (Dosseto et al., 2006a; Dosseto et al., 2006b), Ganges (Granet et al., 2010; Granet et al., 2007), 76 Mackenzie (Vigier et al., 2001), Murray-Darling (Dosseto et al., 2006c). In order to improve our 77 understanding of the controls on the sediment weathering age, here we focus on small 78 catchments draining granitic (Rio Sabana and Rio Icacos) and volcaniclastic lithologies (Rio 79 Mameyes) in tropical Puerto Rico and granitic lithologies in temperate southeastern Australia 80 (Nunnock River). These catchments were chosen because Rio Sabana and Icacos are underlain 81 by similar parent rock as the Nunnock River but characterised by a much wetter and warmer 82 climate, thus allowing us to study the role of climate on weathering age. Furthermore, the Rio 83 Mameyes catchment has similar climatic features to Rio Sabana and Icacos catchments such 84 that the role of lithology can be ascertained. 85 Study areas 86 Nunnock River basin, New South Wales, Australia 87 The Nunnock River is a tributary of the Bega River (southeastern Australia), which drains 88 the eastern flank of the Great Dividing Range into the Tasman Sea (Fig. 2a). This relief formed as 89 a result of the initiation of rifting that opened the Tasman Sea approximately 80 Ma ago (Ollier, 90 1982) and has been slowly propagating inland since then. Altitude ranges 1100 m at the source 91 of the Nunnock River to 220 m at the confluence with the Bega River.