Scientific objectives

The objectives for Expedition 359 relate to two challenges expressed in the IODP Initial Science Plan. The first addresses the variations in regional monsoon systems over multimillion-year timescales. The second addresses the question of how ice sheets and sea level respond to a warming climate, reducing the uncertainties in our understanding of the magnitude and rate of past sea level change. The specific objectives of Expedition 359 are as follows.

1. To decipher the record of Neogene environmental changes in the Maldives sediment archive.

The investigated environmental changes are primarily sea level and current evolution recorded in the prograding carbonate platform margins and drift sequences, respectively. Betzler et al. (2000) show that the major Cenozoic carbonate banks of the Bahama Bank (Atlantic Ocean; ODP Leg 166) and the Queensland Plateau (northeast Australia; ODP Leg 133) recorded sea level changes and synchronous oceanographic and atmospheric circulation events through compositional and architectural changes. Isern et al. (2004), Eberli et al. (2010), and John et al. (2011) argue for the combined effect of eustacy and variation in ocean currents determining the Miocene Marion Plateau stratigraphic record (ODP Leg 194). Previously, the Maldives platform evolution was ubiquitously related to sea level changes (Belopolsky and Droxler, 2004a, 2004b). This model has recently been challenged based on new seismic and hydroacoustic data sets (Figure F4) that show that the carbonate edifice contains major sediment drift bodies, indicating that currents (i.e., environmental changes) were a major driver of its evolution (Betzler et al., 2009, 2013a, 2013b; Lüdmann et al., 2013).

The global middle Miocene climatic optimum was followed by late middle Miocene cooling (Zachos et al., 2001), a reorganization of ocean circulation (Woodruff and Savin, 1989; Flower and Kennett, 1994), deepwater cooling (Lear et al., 2000), and substantial growth of the East Antarctic Ice Sheet (Lewis et al., 2007). Indonesian throughflow changes associated with the Neogene evolution of the Indonesian Gateway and closure of the Tethyan Seaway resulted in reduced atmospheric heat transport from the tropics to the high latitudes in the Indian and Pacific Ocean realms (Flower and Kennett, 1994). Enhanced uplift of the Himalayan region (Clift et al., 2008) concurs with the installation of the seasonally reversing Indian monsoon, resulting in coastal upwelling and increased terrigenous influx to the Indian Ocean (Kroon et al., 1991; Rea, 1992; Zheng et al., 2004). In addition, fluctuations in polar ice volume apparently controlled eustatic sea level variations (Miller et al., 1991, 2005, 2011).

Expedition 359 in the Maldives is designed to recover this mostly unread Indian Ocean archive of the evolving Cenozoic icehouse world from the thick carbonate edifice above Eocene volcanic basement. The Maldives, with its open ocean setting, is a key area for recording this global evolution. Proposed Sites MAL-01 through MAL-07 in the Inner Sea of the Maldives are arranged into two transects from platform to hemipelagic periplatform settings (Figure F2). Lithology and age data from the cores will record changes and turnovers of the neritic carbonate factories and link these changes to paleoenvironmental and paleoceanographic signals recorded in hemipelagic and pelagic series. Changes of the neritic carbonate factory will allow reconstruction of paleoenvironmental changes such as variations in the trophic structure of the water column or sea level changes. Horizons with subaerial exposures of platform carbonates will document times of sea level lowering. The strata in the distal parts of the transects will provide data for dating the changes observed in the platform, applying the usual combination of biostratigraphy and paleomagnetics.

2. To place the Maldives current system into the larger scale ocean current framework present during Neogene global cooling and monsoon evolution.

Mounded drift sequences are the sedimentary bodies deposited by ocean currents acting in the Maldives. Sequence boundaries and internal changes in the arrangement and geometry of reflection patterns attest for changes in current activity and strength through time. Cores from drill holes positioned in the drifts will add direct sedimentological evidence to the seismic-derived model of changing current patterns proposed in Lüdmann et al. (2013). Downhole logging and physical property data measured on the cores are crucial elements for linking seismic geometries with core data. In addition, higher frequency changes will be extracted through bulk sediment grain-size analysis, supported by analysis of grain-size variations of the sortable silt. The tight core coverage will result in an exact age model for these changes so that they can be compared and correlated to Indian Ocean–wide (Gourlan et al., 2008) and global changes in circulation. Because the current system is directly linked to the monsoon system, the drift sequences are expected to provide an extensive record into monsoon history from its onset in the middle Miocene to the present.

3. To obtain a continuous carbon isotopic record to calibrate a platform and platform margin record with the pelagic record.

A major transfer of carbon from the organic to inorganic reservoirs characterizes the Neogene. This transfer is manifested in the large change in the δ13C of oceanic carbonates documented in various records (Shackleton, 1985; Zachos et al., 2001), which has been modeled by various workers (Kump and Arthur, 1999). Carbon isotopic records from carbonate sediments and their associated organic material offer perhaps the best opportunity for the reconstruction of the global carbon cycle (Hayes et al., 1999; Kump and Arthur, 1999). Although open oceanic pelagic records offer the best records for reconstruction of the global carbon cycle (Shackleton, 1985), for the ancient history of Earth we are forced to rely on records of platform-derived sediments (Veizer et al., 1999). It is therefore particularly important to calibrate δ13C in periplatform settings against oceanic records over the same time period. It has been shown that global correlation of δ13C can exist between different carbonate margins that are not correlated to a global carbon cycle (Swart, 2008). The proposed drilling offers the opportunity to obtain a complete record through the Miocene that can be correlated to adjacent ODP sites (i.e., Leg 115 sites) and notable carbon isotopic events (i.e., Monterey carbon excursion) that occurred during the Neogene. The Indian Ocean experienced a complicated history of changes in mass carbonate accumulation related to production during the Miocene, as well as changes in the carbonate compensation depth (Backman, Duncan, et al., 1988; Peterson and Backman, 1990); therefore, such a comparison will enable precise correlation between records such as those obtained from ODP Site 709 (Baker et al., 1990) and Expedition 359 sites. The results will enable an assessment of the competing influences of source and diagenesis relative to oceanic changes in the control of the carbon isotopic composition or the organic and carbonate components.

4. To reconstruct the Cenozoic paleoclimate of the Indian Peninsula.

The onset and intensification phases and the complete Cenozoic evolution of the Indian monsoon remain controversial. There is a lack of consensus regarding the onset or intensification of the Indian monsoon (e.g., Allen and Armstrong, 2012). Some proxy records suggest that the initial intensification occurred at ~7 to 8 Ma (e.g., Kroon et al., 1991; Prell et al., 1992), whereas others suggest a considerably earlier onset, as early as ~22 Ma (Clift et al., 2008; Guo et al., 2002) or the Eocene (Licht et al., 2014). Alternatively, events at ~8 Ma were linked to global cooling (Gupta et al., 2004) or coupled to productivity changes (Filippelli, 1997). Similarly, little consensus exists on the ultimate forcing of monsoon winds and precipitation at orbital timescales (e.g., An et al., 2011; Clemens et al., 2010; Ruddiman, 2006). Detailed long-term monsoon records over the Indian peninsula, south of the Himalayas, are essentially nonexistent. Proposed Site KK-03B will provide the opportunity to reconstruct colocated oceanic and terrestrial records at a location free of any direct influence from the glaciated Himalaya and Tibet. This will allow deciphering the effects of tectonics and premonsoonal/monsoonal climate change on erosion, weathering, and runoff. The final goal is to establish the sensitivity and timing of changes in monsoon circulation relative to external insolation forcing and internal boundary conditions over the Cenozoic. Another important objective for Site KK-03B is to determine the expression of the Paleogene/Eocene Thermal Maximum in a foraminifer-rich pelagic setting in the northern Indian Ocean. Fulfilling this task will depend on the drilled depth and yet-unknown Eocene sedimentation rates.

Relationship with previous drilling

Stratigraphic data for industrial Wells NMA-1 and ARI-1 and Site 716 are presented in Purdy and Bertram (1993), Aubert and Droxler (1996), Belopolsky and Droxler (2004a), and Rio et al. (1990). These sources provide the chronostratigraphic framework for the seismic interpretation. Carbonate lithofacies, paleobathymetric evaluations, and biostratigraphic age determinations based on cuttings and sidewall core analysis for Shell exploration Well ARI-1 were first published by Aubert and Droxler (1996). A vertical seismic profile is used for time-depth conversion and to tie well data to seismic data (Figure F3). For Site 716, which is covered by two high-resolution seismic lines (Figures F3, F4), a simple time-depth estimation is made based on existing whole-core P-wave velocity measurements.

Site 716, drilled in the Inner Sea, recovers the topmost DS4 to DS9. The 264 m thick periplatform ooze bears a mixed record of sea level and bottom-current velocity changes (Lüdmann et al., 2013; Betzler et al., 2013b); grain-size variations for the last 7 My correlate with DS boundaries. Further, the amount of the fine fraction varies, providing insights into past bottom-current speed fluctuations.

For the last 2 My, coarser deposits formed at times (~1.5–0.34 Ma) of an intensified monsoon regime (Betzler et al., 2013b), as presented in Zhisheng et al. (2011). During the early mid-Pleistocene transition, the first large Pleistocene sea level fall triggered deposition of a grain-flow interval in the Inner Sea; subsequently, a strong bottom current led to winnowing of fines at the seafloor. With weakening of the monsoon system around marine isotope stage (MIS) 10, Site 716 grain-size variations become positively correlated with oxygen isotope data and aragonite contents, resembling the sawtooth pattern of Late Pleistocene glacial–interglacial cycles. This reflects highstand shedding from the shallow banks (Paul et al., 2012; Betzler et al., 2013b).

For deposits older than 2–3 Ma, the published record of monsoon strength is patchy and in part contradictory (Kroon et al., 1991; Rea et al., 1992; Clift et al., 2008). Therefore, only a limited and preliminary comparison of Site 716 grain-size data with monsoon proxies is feasible (Figure F7). This comparison indicates that bottom current strength was possibly decoupled from the monsoon system and that the Maldives drift sequences may record variations in the intermediate water flow (Lüdmann et al., 2013). Data indicate that current speed rose between 8 and 5 Ma and was high from 5 to 3 Ma.

In summary, sedimentological analysis of Site 716 shows that periplatform ooze not only records highstand shedding of the atolls but also fluctuations in the bottom current regime, further implying that periplatform ooze, which accumulated in sediment drifts, contains a potential multivariable record of fluctuations of past changes.

The Maldives, located close to India and directly influenced by the monsoon climate, are expected to provide extensive insight into the monsoon history starting from its onset in the middle Miocene to the present. This expedition forms an important complement to work proposed for monsoon-dedicated IODP expeditions. In contrast to these sites at gravity-controlled siliciclastic continental margin settings, the drift deposits of the Maldives represent a continuous time record not disrupted by intercalated mass flow events or major hiatuses, as shown by Site 716. A combined look at independent settings may allow us, for the first time, to receive a complete record of past monsoon variability and contemporaneously help to obtain a better understanding of global climate changes.