Effects of oceanic circulation and volcanic ash-fall on calcite dissolution in bathyal sediments from the SW Pacific Ocean over the last 550 ka
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). Understanding the calcite dissolution processes is of major importance because the balance between the influx of alkalinity from rivers and the loss by oceanic burial of calcium carbonate can influence atmospheric CO2 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 undersaturated waters. Nevertheless, Milliman et al. (1999) demonstrated that considerable calcium carbonate dissolution occurs well above the chemical lysocline, owing to biologically mediated processes 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 Atlantic and the Arabian Sea (Schiebel et al., 2007). Pore-water dissolution also takes place below the sediment/water interface and is driven by the CO2 produced by the degradation of organic matter in the sediments (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 increases with the age of the water mass. In addition, abrupt calcite dissolution 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 calcifying organisms (e.g., Jones and Gislason, 2008, Wall-Palmer et al., 2011).
Changes in calcite dissolution have been intensively studied through several dissolution indices, which are commonly based on calcareous micro-organisms, particularly planktonic foraminifera and calcareous nannofossils (e.g., Dittert and Henrich, 2000, Conan et al., 2002, Mekik et al., 2002, Mekik et al., 2010, Loubere and Chellappa, 2008). This study uses several microfossil dissolution indices from core MD 97-2114 to evaluate the effects of both the oceanic circulation, driven by climate, and the fall-out of volcanic ash on calcite dissolution on the northern slope of the Chatham Rise (Southwest Pacific Ocean) over the last 550 ka. This area is particular interesting because it is influenced by all the major oceanic deep water masses (i.e., North Atlantic Deep Water (NADW), Circumpolar Deep Water (CDW), Antarctic Bottom Water (AABW) and Pacific Deep Water (PDW)). The region has also been affected by episodes of explosive volcanic activity throughout the entire Pleistocene (Wilson et al., 1995, Alloway et al., 2005), with tephra layers well preserved in the marine sedimentary records (Carter et al., 1995, Allan et al., 2008).
The new dissolution proxy data are integrated with previously published data from this core (δ18O and δ13C measured on benthic foraminifera Uvigerina peregrina tests; the percentage of planktonic foraminifera — P%; the benthic foraminiferal epifaunal/infaunal ratio; Lupi et al., 2008, Lupi, 2009, Cobianchi et al., 2012, Mancin et al., 2013), and compared with the dissolution indices available for the nearby ODP sites 1125 (Schaefer et al., 2005) and 1123 (Crundwell et al., 2008). The comparison between MD 97-2114 and the ODP sites, collected at different water depths, is used to reconstruct the history of calcite dissolution/preservation north of Chatham Rise as a response to changes in global climate, regional ocean circulation and volcanic activity. The goals are: i) to describe the glacial–interglacial (G–I) changes in calcite preservation at different water depths during the last 550 ka; ii) to identify the main short-term calcite dissolution events, distinguishing those driven by climate and ocean circulation from those driven by the local activity of the Taupo Volcanic Zone; iii) to discuss possible mechanisms for the dissolution events recorded at different depths on the northern Chatham Rise.
Section snippets
Regional setting
The modern surface circulation of the area is influenced by the Subtropical Front (STF), which is locked to the southern flank of Chatham Rise (Fig. 1B). The surface waters above the MD 97-2114 site are warm, high-salinity, macronutrient-poor, micronutrient-rich Subtropical Water (STW) (Table 1). ODP 1125 is also overlain by STW, although episodic incursions of Subantarctic Water (SAW), via the Mernoo Saddle, have been documented during the glacials (Schaefer et al., 2005). Similarly, ODP 1123
Core location and description
The IMAGES core MD 97-2114 (42°22′27″S; 171°20′42″W) was recovered, in May 1997 during the IMAGES III cruise of the R/V Marion-Dufresne, from the north-eastern slope of the Chatham Rise at a water depth of 1936 m (Fig. 1). The age model was determined from stable isotopes and documents a continuous sedimentary record of the past 1.07 Ma, with an average accumulation rate of 2.6 cm/ka (Cobianchi et al., 2012 and references therein). For this study, the interval Marine Isotope Stage 1–13
Identification of the tephra layers and correlation with the site ODP 1123
The studied portion of the core MD 97-2114 (ca. 16 m long, recording the last 550 ka) contains at least 11 macro- and microscopic tephra layers, which are identified on the basis of high concentrations of glass shards, coupled with a proportionate decrease of foraminiferal tests (Fig. 2).
Within the macroscopic and thicker tephra, the glass fragments can reach up to 100% of the 63–150 μm fractions (e.g., T2 and T3 layers at ca. 350 ka), while in the cryptotephras the glass shards usually contribute
Long-term changes in carbonate dissolution
The smoothed FI% of the core MD 97-2114 shows a long-term (timing exceeding the G–I cycle) increasing trend from MIS5 upwards (Fig. 5), confirmed by the GAM analyses (Supplementary data, Fig. A). This trend probably reflects increased dissolution at the sea floor with the local development of the oxygen minimum zone (OMZ) (Hayward et al., 2004), which currently intersects the depth of our core site (McCave et al., 2008). Alternatively the OMZ has shifted depth in the water column during this
Conclusions
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There is a general increase in dissolution in this region from MIS5 to present, possibly due to the development of a minor OMZ in this region.
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Pacific style G–I CaCO3% dissolution events are evident at MD 97-2114 from all the different calcite dissolution proxies used in this study, with maximum preservation during deglaciations and maximum dissolution during the glaciations. This is the result of increasing PDW during the interglacials, and increased ventilation of the deep waters by AABW
Acknowledgements
The IMAGES core MD 97-2114 was recovered in May 1997 during the IMAGES III cruise of the R/V Marion-Dufresne. We would like to thank the captain and crew and the scientists involved in the voyage that collected the core and particularly E. Michel, IMAGES chief scientist of the 1997 cruise, who allowed us to study core. Bruce Hayward is acknowledged for the critical reading of the manuscript. We would like to thank Roberto Sacchi, Assistant Professor at the Pavia University, for his invaluable
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