Sedimentation and burial of organic and inorganic temperature proxies in the Mozambique Channel, SW Indian Ocean Ulrike Fallet a,n , Isla S. Castan ˜eda b,1 , Aneurin Henry-Edwards a , Thomas O. Richter a , Wim Boer a , Stefan Schouten b , Geert-Jan Brummer a,c a NIOZ, Royal Netherlands Institute for Sea Research, Department of Marine Geology, P.O. Box 59, NL-1790 AB Den Burg, The Netherlands b NIOZ, Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, P.O. Box 59, NL-1790 AB Den Burg, The Netherlands c Faculty of Earth- and Life Sciences, VU University Amsterdam, de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands article info Article history: Received 15 December 2010 Received in revised form 12 October 2011 Accepted 13 October 2011 Available online 6 November 2011 Keywords: Mozambique Channel SW Indian Ocean Temperature proxies Biomarkers U k 0 37 TEX 86 Foraminifera d 18 O Mg/Ca Radionuclides ( 234 Th, 210 Pb, 14 C) Sediment Bioturbative mixing Sediment trap abstract Paleoceanographic studies strongly rely on proxies to reconstruct past environmental conditions. However, several factors influence the reliability of these proxies, particularly during sedimentation and burial. In this study, we measured both inorganic (d 18 O and Mg/Ca on three species of planktonic foraminifera (Globigerinoides ruber, Globigerinoides trilobus and Neogloboquadrina dutertrei) and organic (U k 0 37 and TEX 86 ) sea surface temperature (SST) proxies on core top sediment taken from an east-west transect across the Mozambique Channel. We contrast our findings with previously published modern time-series temperature proxy data from a sediment trap moored in the mid-channel. The coretop sediment was analyzed for 14 C, 210 Pb and excess (xs) 234 Th. While 234 Th xs data indicate a flux of fresh particulate matter to the bottom sediment, radiocarbon dating shows that the core top sediments are composed of material that is on average about 1000 years old. The fine organic carbon is consistently (even though only slightly) younger than the coarser foraminiferal calcite, which is likely caused by preferential downcore mixing of the fine fraction. Besides vertical mixing by bioturbation, stable lead isotope ratios from the time-series particle fluxes indicate episodic lateral transport of old particles from the shelf to the deep Mozambique Channel as an additional source of pre-aged material in core tops. Core top temperature proxies show warmer values close to the channel flanks while colder values are found in the mid-channel. These could be associated with higher maximum summer temperatures in modern coastal waters in contrast to the mid-channel. Additionally, we find an offset in all foraminiferal proxies between core top samples and time-series data that corresponds to 1–3 1C, which probably reflects climate variability over the past  2000 years. However, this temperature difference is not observed in the organic proxies U k 0 37 and TEX 86 , which may result from current transport of unconsolidated organic matter that can homogenize the organic proxy signal prior to burial. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction In order to understand past ocean and climate systems, proxies are needed that accurately record environmental conditions (Elderfield, 2002; Lea, 2003). However, many factors can influence proxy signals during sedimentation and burial. One important aspect is the contribution of pre-aged material to the sedimentary record (Gardner and Sullivan, 1981; Laine et al., 1994) and the time- averaging effect of bioturbation (Keigwin and Guilderson, 2009). Proxy parameters from core top sediments are frequently used for calibration purposes to modern oceanographic parameters in the absence of sediment trap series (Dekens et al., 2002; Kim et al., 2008; M¨ uller et al., 1998). Yet, calibration of pre-aged allochthonous and bioturbated sediment proxies to modern SST can potentially intro- duce uncertainties or biases. Furthermore, sedimentary records con- taining variable contributions of pre-aged material can result in pooled proxies that do not reflect the environmental conditions at their original place of deposition (Benthien and M ¨ uller, 2000). Upward bioturbation of older sediment from the deeper core and downward bioturbation of fresh material from the top play an important role in time-averaging proxy-temperature in sedi- ments. Local variability in the intensity of bioturbation in deep- sea sediment may be as important as regional differences in sediment properties (de Master and Cochran, 1982). Bioturbation of sediment usually reaches as deep as 10 cm (Boudreau, 1998), which implies a potentially substantial age bias in proxy signals particularly in deep sea sediments where sedimentation rates are usually low. Other studies concluded that mixing depth is not constant on a global scale, but varies as a predictable function of particulate organic carbon flux (Smith and Rabouille, 2002; Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/dsri Deep-Sea Research I 0967-0637/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2011.10.002 n Corresponding author. E-mail address: ulrike.fallet@nioz.nl (U. Fallet). 1 Present address: Department of Geosciences, University of Massachusetts Amherst, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003, USA. Deep-Sea Research I 59 (2012) 37–53