Effects of oceanic circulation and volcanic ash-fall on calcite dissolution in bathyal sediments from the SW Pacific Ocean over the last 550 ka M. Cobianchi a, ⁎, N. Mancin a , C. Lupi a , M. Bordiga a,b , H.C. Bostock c a Dipartimento di Scienze della Terra e dell'Ambiente, Università degli Studi di Pavia, Via Ferrata 1, 27100 Pavia, Italy b Department of Earth Sciences, Uppsala Universitet, Villavägen 16, 75236 Uppsala, Sweden c National Institute of Water and Atmospheric Research, Greta Point, Wellington, New Zealand abstract article info Article history: Received 19 September 2014 Received in revised form 9 March 2015 Accepted 29 March 2015 Available online 16 April 2015 Keywords: Carbonate dissolution Fragmentation index Nannofossil Dissolution Index Late Pleistocene SW Pacific The effects on calcite dissolution of both oceanic circulation and volcanic ash-fall were evaluated in lower bathyal sediments over the last 550 ka record from core MD 97-2114, recovered on the northern slope (depth of 1936 m, in the Pacific Deep Water, PDW) of the Chatham Rise (east of New Zealand, SW Pacific Ocean). This area has been impacted by changes in glacial/interglacial circulation and ocean chemistry as well as by the explosive volcanic activity of the Taupo Volcanic Zone. Several micro-paleontological dissolution proxies, based on planktonic forami- nifera and calcareous nannofossils, were analysed in order to evaluate the calcite dissolution of the deep-sea sedi- ments. These were compared with a couple of proxies of primary productivity (benthic foraminiferal epifaunal/ infaunal ratio and δ 13 C benthic foraminifera ) and the abundance of volcanic glass. The dissolution proxy data from MD 97-2114 were compared with two nearby ODP sites, ODP 1123 (3290 m deep, bathed by the lower Circumpolar Deep Water, LCDW) and ODP 1125 (1365 m deep, bathed by the Antarctic Intermediate Water, AAIW). The results suggest: (1) the calcite dissolution/preservation cycles at all three core sites show Glacial–Interglacial (G–I) periodicities that match the previously described “Pacific-style” CaCO 3 cycles; (2) several short-term disso- lution events do not follow this general scheme and occur following tephra deposition. The dissolution related to the tephra deposition seems to have mostly affected calcareous nannofossils, thus we hypothesise that the ash- fall induced a temporary reduction of the surface water pH (below 7.8), which affected the coccolithophores that inhabit the surface waters. (3) Other short-term dissolution events (1000 years) are unrelated to tephra deposi- tion and are possibly driven by the slowing of deep-sea circulation and a reduced Deep Western Boundary Cur- rent (DWBC). This lead to the dominance of older, more corrosive Pacific Deep Water (PDW) flowing in to the region, resulting in coeval dissolution episodes at all three core sites (depth range from 1365 to 3290 m). © 2015 Elsevier B.V. All rights reserved. 1. Introduction The concentrations of calcareous pelagic ooze, that drapes the oceanic floor at bathyal depths, are the result of three processes: i) biogenic calcite production in the surface waters; ii) dilution from periplatform carbonate, pelagic non-carbonate or terrigenous sediments and, iii) dissolution in the water column or at the sea floor (e.g., Broecker, 2003; Ridgwell and Zeebe, 2005; Mekik et al., 2010). Un- derstanding the calcite dissolution processes is of major importance be- cause the balance between the influx of alkalinity from rivers and the loss by oceanic burial of calcium carbonate can influence atmospheric CO 2 over glacial–interglacial cycles (Hales and Emerson, 1996). Calcite dissolution in the oceans takes place in several different settings: in the water column, at the sediment/water interface and within the sediment pore-water (e.g., Broecker, 2003). In the water column and at the sea floor, dissolution occurs typically when calcite shell/skeleton sinks below the lysocline depth and is exposed to un- dersaturated waters. Nevertheless, Milliman et al. (1999) demon- strated that considerable calcium carbonate dissolution occurs well above the chemical lysocline, owing to biologically mediated pro- cesses within flocculates and aggregates. Dissolution of foraminifera in the “Twilight zone” from 100 to 1000 m, well above the lysocline, has also been demonstrated from plankton tows in the North Atlan- tic and the Arabian Sea (Schiebel et al., 2007). Pore-water dissolution also takes place below the sediment/water interface and is driven by the CO 2 produced by the degradation of organic matter in the sedi- ments (Hales and Emerson, 1996; Schiebel et al., 2007). Another mechanism that can produce calcite dissolution on the sea floor is the “aging effect” (e.g., Anderson et al., 2008; Russon et al., 2009). This effect arises through the progressive addition of re- mineralized organic carbon from the overlying water column that in- creases with the age of the water mass. In addition, abrupt calcite disso- lution events could be related to the fall-out of volcanic ash in the ocean, which has been demonstrated to lower pH in the surface waters, resulting in a reduction of carbonate ions and impacts planktonic Palaeogeography, Palaeoclimatology, Palaeoecology 429 (2015) 72–82 ⁎ Corresponding author. Tel.: +39 0382985897; fax: +39 0382985890. E-mail address: miriam.cobianchi@unipv.it (M. Cobianchi). http://dx.doi.org/10.1016/j.palaeo.2015.03.045 0031-0182/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo