Scientific objectives

The three drilling regions will recover sediment sections variously spanning Late Cretaceous through Holocene (Fig. F1A), depending on the location.

Pliocene–Pleistocene objectives

Pliocene–Pleistocene objectives include reconstructing salinity changes as well as the erosion and runoff signals in the Bay of Bengal and Andaman Sea in order to

  • Establish the sensitivity and timing of changes in monsoon circulation relative to insolation forcing, latent heat export from the Southern Hemisphere, global ice volume extent, and greenhouse gas concentrations;
  • Determine the extent to which Indian and East Asian monsoon winds and precipitation are coupled and at what temporal and geographic scales;
  • Better separate the effects of climate change and tectonics on erosion and runoff; and
  • Provide verification targets for climate models: the majority of current atmosphere–ocean general circulation models do not accurately simulate the spatial or intraseasonal variability of monsoon precipitation.

Deep-time objectives

Deep-time objectives include the following:

  • To understand the timing and conditions under which monsoonal circulation initiated and reconstruct the variability of the Indian monsoon at orbital timescales;
  • To unravel the relationship between Indian monsoon variability and major past global climatic events such as the Oligocene/Miocene cooling (Zachos et al., 1997), the onset of the mid-Miocene Climatic Optimum (Holbourn et al., 2007; Zachos et al., 2001), mid-Miocene cooling and Antarctic cryosphere expansion (Holbourn et al., 2013), and the Pliocene–Pleistocene enhancement of Northern Hemisphere glaciation (Lisiecki and Raymo, 2005, 2007);
  • To establish a complete Oligocene–present astronomically tuned timescale based on high-resolution benthic and planktonic isotope reference curves for the Indian Ocean; and
  • To incorporate high-resolution distribution studies of well-preserved Oligocene–recent calcareous and siliceous microfossils from the Indian Ocean into global compilation studies of paleoclimatic and biotic evolution.

Chronology, proxies, and tracers

The initial chronostratigraphic reference frame will consist of geomagnetic polarity reversal stratigraphy and nannofossil and foraminiferal biostratigraphy in combination with siliceous microfossil zonation. Using this stratigraphic framework and composite sections from double or triple coring at each site, high-resolution benthic δ18O records will be created and correlated to the global marine benthic δ18O chronology of the past 5 m.y. (Lisiecki and Raymo, 2005) and to orbitally tuned Pacific records for deeper-time intervals (e.g., Holbourn et al., 2013).

An array of physical, biological, chemical, and isotopic proxies is available to assess changes in runoff and weathering signals associated with monsoonal precipitation and changes in monsoon-related paleoceanographic parameters such as changes in surface productivity, water column stratification and vertical mixing, ventilation, and the presence of externally sourced intermediate waters. The following proxy indicators are essential for these reconstructions.

Salinity indicators

The composition of core-top planktonic foraminiferal assemblages in the Arabian Sea and the Bay of Bengal are related to surface salinity. In particular, high percentages of Neogloboquadrina dutertrei are associated with low surface salinities (Cullen, 1981). Similarly, morphological variations of Emiliania huxleyi provide constraints on seawater salinity for the Late Pleistocene record (Bollmann and Herrle, 2007), whereas the process length of the dinoflagellate cysts of Lingulodinium machaerophorum provides salinity information back to the Oligocene (Mertens et al., 2009).

A number of investigators have employed various combinations of sea-surface temperature (SST), planktonic δ18O, and sea level reconstructions to derive δ18O of seawater (δ18Osw) as a proxy for salinity (Ahmad et al., 2008; Bahr et al., 2011; Billups et al., 2002; Govil and Naidu, 2011; Holbourn et al., 2010; Lea et al., 2000; Rashid et al., 2007, 2010; Steinke et al., 2010). These proxies have been utilized to derive records of changing sea-surface salinity across timescales ranging from millennial to orbital and tectonic. In most applications, Mg/Ca ratios in planktonic foraminifer shells (or alkenone Uk′37) have been used to derive SSTs independent of δ18O (e.g., Dekens et al., 2008; Elderfield and Ganssen, 2000; Nürnberg et al., 1996). Application to Bay of Bengal and Andaman Sea cores demonstrates the utility of this proxy in the northern Bay of Bengal (Govil and Naidu, 2011; Kudrass et al., 2001; Rashid et al., 2007, 2010). The paired SST–planktonic δ18O approach is also being applied to individual shells (Haarmann et al., 2011; Khider et al., 2011). Although resource intensive (~50 analyses per sample to achieve reliable statistics), this approach ensures capture of a robust seasonal signal that is associated closely with foraminifers calcifying during the summer monsoon salinity minima. This could play a key role in untangling the debate surrounding the divergent interpretation of summer monsoon proxies (e.g., Clemens et al., 2010; Clemens and Prell, 2007; Ruddiman, 2006; Wang et al., 2008). Local or regional δ18Osw-salinity relationships for the Bay of Bengal (Delaygue et al., 2001; Singh et al., 2010) are available to derive even more direct salinity estimates at appropriate timescales (e.g., Fig. F14).

Runoff and erosion indicators

The impact of monsoonal precipitation on chemical weathering and transport is recorded in the physical, chemical, and isotopic composition of clastic sediments in the Bay of Bengal and the Arabian Sea. The delivery of terrigenous material to the Bay of Bengal is recorded even at distal locations (Site N90E-2C), where delivery increased in the middle Miocene with two subsequent pulses at ~7.0–5.6 and ~3.9–2.0 Ma (Hovan and Rea, 1992). These increases were interpreted to represent variations in the fluvial flux resulting from the uplift and erosion of the Himalaya, although they could equally represent intensifying erosion driven by strengthening monsoonal precipitation. An independent climate record of rainfall is needed to determine the trigger for changes in erosion patterns and rates. Indeed, it is noteworthy that the timings of these erosional pulses are broadly coincident with large-scale hydrological changes observed in the Himalayan foreland basin and Arabian Peninsula (Huang et al., 2007). Strong north–south elemental and isotopic gradients in the surface waters also reflect continental erosion and fluvial transport processes that are similarly recorded in underlying sediments.

Carroll et al. (1993) and Moore (1997) recognized the Ganga-Brahmaputra River system as a significant source of Ba to the global ocean. Figure F15 illustrates this strong Ba-salinity relationship in the Bay of Bengal. More recent studies have established the Ba/Ca ratio of foraminifer CaCO3 as a proxy for river runoff. Ba/Ca ratios in planktonic foraminifer shells have been utilized to monitor river input to marine sections in the Arctic and Atlantic Oceans (Hall and Chan, 2004; Weldeab et al., 2007a, 2007b) as well as the Mediterranean (Sprovieri et al., 2008). This approach assumes that the Ba/Ca ratio in planktonic foraminifer shells is dominated by the Ba/Ca concentration of seawater rather than other factors. The results of Hönisch et al. (2011) strongly support this assumption; environmental parameters including pH, temperature, salinity, and symbiont photosynthesis do not appear to affect Ba substitution into planktonic foraminiferal calcite.

The utility of Sr and εNd proxies for monitoring the weathering signal from rivers in the Bay of Bengal, the Arabian Sea, and other locations has been well documented (e.g., Burton and Vance, 2000; Clift and Plumb, 2008; Colin et al., 1999; Goswami et al., 2012; Osborne et al., 2008; Padmakumari et al., 2006; Rahaman et al., 2009; Stoll et al., 2007; Tripathy et al., 2011). The Nd isotope composition of the carbonate component of Bay of Bengal sediments has been used to investigate variations in the relative contribution of discharge from the Ganga-Brahmaputra, Irrawaddy, and Arakan Rivers (Burton and Vance, 2000; Gourlan et al., 2008, 2010; Stoll et al., 2007). These studies agree that input from the Ganga-Brahmaputra River decreased during glacial periods, which is consistent with decreased Indian summer monsoon strength. The Nd isotope ratio of surface seawater near the proposed Andaman Sea sites reveals the influence of nonradiogenic inputs from the Ganga-Brahmaputra and Irrawaddy Rivers; εNd ranges from –11.4 in the Andaman Sea to –9.9 near Site N90E-2C (Amakawa et al., 2000). At the Indian margin, the influence of the Deccan and Indian craton should also be important (Goswami et al., 2012). The Sr-Nd isotope composition of the silicate fraction will provide information on temporal variations in provenance and temporal variations in weathering and runoff (Colin et al., 1999; Goswami et al., 2012; Rahaman et al., 2009; Tripathy et al., 2011).

The very narrow east Indian margin is an excellent environment for the application of elemental and mineralogical analysis of terrigenous sediments in order to constrain variations in the intensity of chemical weathering as well as provenance changes. The combination of high river flow and narrow margins promotes the rapid movement of materials across the shelf to the slope, reducing issues associated with prolonged storage (Ponton et al., 2012; Sridhar et al., 2008) and simplifying the source-to-sink process and interpretation of the marine geochemical record.

The use of nondestructive X-ray fluorescence (XRF) core scanning measurements enables rapid, high-resolution elemental analyses of core sections (e.g., Mulitza et al., 2008). Calibration studies of core-top sediments have shown that different elemental ratios can be reliably applied to trace different weathering regimes or soil types (Govin et al., 2012). Ratios of mobile versus immobile elements have been used for many years to trace the intensity of chemical weathering in river source areas; high-resolution studies have shown that these ratios are responsive to environmental forcing on a number of timescales within the Asian monsoon system (Liu et al., 2007; Wan et al., 2009). Such proxies are even more effective when combined with traditional clay mineralogy, providing additional information on weathering regimes in support of Nd isotopes and elemental XRF analyses. Variations in clay mineralogy have provided a wealth of information on monsoon strength for the past 280 k.y. in the Bay of Bengal (Colin et al., 2006) and have also been used over longer time periods in the South China Sea (Wan et al., 2006, 2007) as well as on short timescales during the Quaternary in the Indus Basin (Alizai et al., 2012). High-resolution clay mineral records can be derived from spectral analysis of the core as well as through traditional XRF methods, and these have been effective in reconstructing weathering intensities from earlier core records (Clift et al., 2008; Giosan et al., 2002).

Environmental water indicators

Compound-specific leaf-wax D/H (δDwax) has been applied as a proxy for the D/H of precipitation (δDppt) in marine sediments (Huang et al., 2007; Pagani et al., 2006; Schefuß et al., 2011; Sluijs et al., 2006), lake sediments (Hou et al., 2007a, 2007b; Huang et al., 2002; Jacob et al., 2007; Sauer et al., 2001; Shuman et al., 2006; Tierney et al., 2008), and loess sediments (Hou et al., 2008; Liu and Huang, 2005). Sachse et al. (2004) and Hou et al. (2008) defined quantitative links between lake surface sediment δDwax and δDppt across continental-scale precipitation gradients in Europe and North America, whereas Rao et al. (2009) established the same link between soil δDwax and δDppt for eastern China. The North American, European, and Chinese data sets are very consistent with one another, demonstrating the broad applicability of this proxy to monitoring changes in the hydrological cycle. δD of alkenones (Schouten et al., 2006; Vasiliev et al., 2013) offers another possible means of salinity reconstruction through the effect on isotopic fractionation; hydrogen isotopic fractionation of alkenones relative to source water decreases as salinity increases.

The carbon isotopic composition of terrestrial plant biomass is primarily a function of the plant’s specific photosynthetic pathway and the isotopic composition of atmospheric CO2. Leaf wax δ13C records have been used extensively to reconstruct past changes in the balance of C3 versus arid-adapted C4 vegetation (see Feakins et al., 2005, and Ponton et al., 2012, for examples in the monsoon domain). Analysis of both leaf-wax δD and δ13C can be used to distinguish between changes in moisture source (LeGrande and Schmidt, 2010) and/or availability.

Deep biosphere linkages

Specific to Indian Ocean sediments, a number of deep biosphere–related scientific problems can be addressed at the proposed drilling locations as put forth in the final report of Scientific Drilling in the Indian Ocean workshop (​workshop-reports/) including:

  • How has uplift of the Himalayans influenced monsoon and input of terrestrial matter into the Bay of Bengal and the Arabian Sea and impacted the development of the deep biosphere since the Oligocene?
  • How has the drainage from the Himalayan rivers influenced the development of subseafloor community structures and diversity?
  • How has the subseafloor biosphere been inoculated with terrestrial microorganisms (biogeography)? Are there regional differences between the Bay of Bengal and the Arabian Sea?