Paleoproductivity and paleoredox conditions during late Pleistocene accumulation of laminated diatom mats in the tropical West Pacific Zhifang Xiong a , Tiegang Li a, ⁎, Thomas Algeo b , Qingyun Nan a , Bin Zhai c , Bo Lu a a Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China b Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA c Key Laboratory of Marine Hydrocarbon Resource and Geology, Qingdao Institute of Marine Geology, Ministry of Land and Resources, Qingdao 266071, China abstract article info Article history: Received 8 June 2012 Received in revised form 26 September 2012 Accepted 27 September 2012 Available online 5 October 2012 Editor: Carla M. Koretsky Keywords: Diatom mats Redox Paleoproductivity Ethmodiscus rex West Pacific Last Glacial Maximum Paleoproductivity and paleoredox conditions were reconstructed in a sediment core in the Parece Vela Basin of the eastern Philippine Sea. The core consists of three units, from youngest to oldest: (1) laminated diatom mats (LDM) formed by Ethmodiscus rex during the Last Glacial Maximum (~18–28 kyr B.P.), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Elevated levels of export productivity during LDM deposition are indicated by high values for excess Ba, opal content, and TOC/Ti ratios. Estimated rates of organic carbon degradation (ca. 98%), opal mass accumulation (average 1322 g m -2 yr -1 ), and corrected organic carbon flux (average 248 g m -2 yr -1 ) are comparable to high-productivity regions of the modern ocean. The LDM is also characterized by moderate en- richment of redox-sensitive elements such as U, Mo, Cd, and Zn, highly 34 S-depleted pyrite sulfur isotopic compo- sitions (indicating bacterial sulfate reduction in a sulfate-unlimited system), and C–S–Fe systematics reflecting limitation of pyrite formation by organic matter rather than reactive Fe availability. These features suggest mainly suboxic conditions in bottom waters but development of sulfidic–anoxic conditions at or close to the sediment– water interface. Association of intensified anoxia with productivity maxima indicates that export production was a more important control on bottom water redox conditions than lateral ventilation. The DC and PC accumu- lated under oxic to suboxic conditions. Our observations suggest that redox environments during deposition of laminated marine sediments are more complicated and varied than previously thought, and, thus, the use of sediment lamination as an indicator of anoxic bottom water conditions must be approached cautiously. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Although mat-forming giant diatoms are found only sporadically in the global ocean, their role in export production and marine redox changes has drawn wide attention (Kemp and Baldauf, 1993; Kemp et al., 2006; Romero and Schmieder, 2006). These diatoms, including Ethmodiscus rex, Rhizosolenia spp., and Thalassiothrix spp., represent a “shade flora” that is able to grow in low-light subsurface waters and/or to move between a deep nutrient pool and the surface euphotic zone via regulated buoyancy (Villareal, 1993; Kemp et al., 2000). In the tropi- cal ocean, giant diatoms are adapted to a stratified water column, thriv- ing at the interface between sunlit surface waters and nutrient-rich deep waters and creating a deep chlorophyll maximum (DCM) (Kemp et al., 2000, 2006). Kemp et al. (2000) demonstrated that a DCM composed of giant diatoms could generate a “fall dump” that rivaled or exceeded that of the “spring bloom” species, highlighting their importance for ex- port production. Where export production is sufficiently high, underly- ing marine sediments consist of laminated diatom mats (LDMs). Earlier investigations hypothesized that LDMs accumulate in settings with well-oxygenated bottom waters, and that preservation of laminae is due to rapid deposition and the intrinsic meshwork of the mats, which prevents or restricts benthic activity (Kemp and Baldauf, 1993; Bodén and Backman, 1996; King et al., 1998; Pike and Kemp, 1999). This hy- pothesis was presented as a challenge to the paradigm that lamination in sediments provides evidence of oxygen-deficient bottom water condi- tions and suppressed bioturbation (Behl and Kennett, 1996; Bull et al., 2000). However, the evidence for oxic conditions given in these studies was based largely on micropaleontological data, so this hypothesis has not been rigorously tested to date. LDMs produced by giant mat-forming diatoms such as E. rex accumu- lated widely in the eastern Philippine Sea during the Last Glacial Maxi- mum (LGM) (Zhai et al., 2009). The location of the study site below the calcium carbonate compensation depth, or CCD, resulted in a lack of dilu- tion by carbonate sediment, providing an opportunity to investigate changes in productivity and redox conditions that can be linked specifi- cally to these diatoms. In this paper, we report on the sedimentology and geochemistry of LDMs at a site in the Parece Vela Basin of the eastern Philippine Sea in order to (1) reconstruct variations in productivity levels and water-column redox conditions during mat-forming diatom blooms, (2) evaluate the hypothesis that LDM accumulation occurs under oxic Chemical Geology 334 (2012) 77–91 ⁎ Corresponding author. E-mail address: tgli@qdio.ac.cn (T. Li). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2012.09.044 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo