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doi:10.14379/iodp.sp.354.2014

Scientific objectives

We focus on (1) the erosional history of the Himalaya and its bearing on the development of the Himalaya and Tibet as topographic features and (2) the development of the Asian monsoon in Cenozoic time as recorded in the Bay of Bengal.

1. Calibration of Neogene to present changes

Although the Miocene to recent sedimentary records in the Bengal Fan suggest that Himalayan erosion was quite comparable to the actual regime, tectonic and climatic changes have occurred that are both likely to have influenced sedimentation in the Bay of Bengal. This includes upper Miocene changes in accumulation rate, continental vegetation, and weathering intensity that are documented both in the continental basin and the Bengal Fan (e.g., Quade and Cerling, 1995; France-Lanord et al., 1993; Burbank et al., 1993; France-Lanord and Derry, 1994; Martinod and Molnar, 1995; Clift et al., 2008; Galy et al., 2010). The development of intense channel-levee deposition in the Bengal Fan starts appearing in the upper Miocene or Pliocene and represents a major change in sediment delivery to the Bay of Bengal (Schwenk and Spieß, 2009). Later, late Pliocene global cooling that led to the growth of Northern Hemisphere ice sheets was related to an increase of detrital sedimentation rate and grain size (Zhang et al., 2001). Although the reality of this change has been questioned (Willenbring and von Blanckenburg, 2010) the evolution toward higher climate instability should prevent fluvial and glacial basins from reaching an equilibrium state and trigger their erosion. Such changes were observed in the distal Bengal Fan around ~0.8–1 Ma when the 100 k.y. cycle became strong. This expedition will document this critical period in the Earth’s most intense erosion system, as tectonic and climate changes have left signatures in accumulation rates, grain sizes, clay mineralogy, isotopic ratios, organic carbon burial, and so on that can be measured (Cochran, Stow, et al., 1989; Derry and France-Lanord, 1997; France-Lanord et al., 1993).

First, to what extent are variations in accumulation rates, clay mineralogy, and grain sizes from ODP Holes 717–719 (Fig. F5) representative of other parts of the fan? Because those holes only recovered sediments from the most distal parts of the Bengal Fan within a growing syncline, sedimentation might have been affected both by the large distance from the source and by the barrier imposed by the surrounding anticlines. The decreases in accumulation rate and grain size at ~7 Ma and the synchronous increased percentage of smectite (Bouquillon et al., 1990) suggest that if the monsoon strengthened at that time, it apparently did so without creating a more energetic erosive system, as might be expected from the strong seasonal precipitation of the monsoon. Obviously, if we find the same pattern of low accumulation rates, small grain sizes, and a large percentage of smectite at 7–8 Ma in the holes cored at 8°N during Expedition 354, we must consider the possibility that if the monsoon strengthened at that time, it did so by decreasing, not increasing, erosion rates. If we find a sediment history different from that in Holes 717–719, it is possible that the sedimentary record at the distal edge of the fan does not record faithfully the changes input at the source of the fan.

The present-day monsoon, if named originally for the seasonally steady winds over the Arabian Sea, also evokes the image of heavy rain over the Indian subcontinent. We have no evidence to date, and perhaps conflicting evidence, showing that precipitation increased over the Ganga and Brahmaputra drainage basins and the Bay of Bengal at 7–8 Ma (Derry and France-Lanord, 1997; Dettman et al., 2001) in spite of enhanced Indian monsoon seasonality over this period (Zhisheng et al., 2001). Assessing paleomonsoon intensity is challenging and can be tracked with two approaches. First, the efficiency of sediment transport from the Himalaya to the Bay of Bengal is directly controlled by the seasonality and intensity of rainfall that creates high river discharge (Lupker et al., 2011). This transport in turn exerts a control on sediment fluxes to the Bay of Bengal that can be traced by accumulation rates. Second, δ¹⁸O in planktonic foraminifers deposited in the Bay of Bengal in late Quaternary time are related to amounts of precipitation (Duplessy, 1982). If these microfossils can be recovered, they will provide another tool to assess the strength of the monsoon.

The main reason for drilling more than one site at 8°N is to minimize the effect of varying sedimentation rates associated with a pronounced increase in the vicinity of the active channel at any one time and the migration of the active channel and hence to avoid the biases that one site (or a set of adjacent sites) might give. Although the main focus of these sites is on the changes near 7–8 Ma, obviously we will obtain material from a longer period. Our easternmost site (MBF-3A) in the eastern Bay of Bengal will recover material older than 14 Ma, which will allow the kinds of studies described above to test whether the monsoon strengthened during the early Miocene, as proposed by Ramstein et al. (1997) and Fluteau et al. (1999).

In addition, we plan to study other temporal and spatial variations in the sediment over the last 7 m.y. Site 218 and sites cored during Leg 116 show changes that possibly reveal other variations in the erosion regime. Kroon et al. (1991) showed a dip in the abundance of Globigerina bulloides at ~5 Ma, which might indicate a lull in the monsoon, the temporary dominance of another upwelling-sensitive foraminifer (Kroon et al., 1991), or some other inadequacy of Bulloides to measure monsoon strength. We also seek quantitative measures of the interaction between climate change and sedimentation associated with the global cooling and onset of the Northern Hemisphere glaciation at ~2.7–3 Ma and with the change from precession- and obliquity-dominated climate variations to the strong 100 k.y. cycle at 0.8–1 Ma. This latter change appears to be marked in deposition rates, grain sizes, and clay mineralogy in ODP Holes 717–719 (France-Lanord et al., 1993). Again, one objective is to decide how representative the results from Sites 717–719 are. The same proxies for monsoon strength and for erosion will be available for study of this period. The results from Sites 717–719 reveal no evidence of a change in erosion rate at 2.7–3 Ma, in contrast with what might be expected if glacial erosion were important and increased at that time. Moreover, if the only important change in sedimentation rate, and hence presumably in erosion rate, occurred at 0.8–1 Ma, such a change would provide a clue to what kind of change was important. We anticipate being able to resolve temporal variations on the timescale of orbital variations, but obviously we must expect large variations in sedimentation rates. Hence, three sites on our transect (MBF-4A, MBF-5A, and MBF-6A) are proposed to limit the biases on long-term studies associated with penetrating buried channel-levee systems, and another three sites (MBF-1A, MBF-2A, and MBF-3A) will be drilled in between to optimize the temporal sampling of terrigenous sediment input and to analyze the impact of depocenter migration on sedimentary facies and recorded signals.

The 7 Ma transition is also marked by expansion of C4 photosynthetic plants in the Himalayan basin (Quade et al., 1989). Although C4 plant expansion may result from a global decrease in atmospheric pCO₂ (Cerling, 1997), studies suggest that pCO₂ was already low at that period (Pagani et al., 1999; Beerling and Royer, 2011). In the latter hypothesis, C4 plant expansion would instead require an adaptation to more arid conditions in the floodplain. Sediments sampled during Leg 116 show close links among variations in clay mineralogy (smectite/illite ratio), in total organic carbon, and in δC (C3 versus C4 plants) (Fig. F5) (France-Lanord and Derry, 1994; Freeman and Colarusso, 2001). These relationships suggest changes in sediment provenance, with a mountain end-member delivering material unaltered with low organic carbon content of C3 type and a floodplain end-member delivering altered material with high organic carbon content of C4 type. If confirmed by new drilling at the scale of the whole fan, such relations would favor the hypothesis of a regional climate change toward dryer conditions.

2. Forcing of the carbon cycle and climate

Drilling the Bengal Fan should allow investigation of the effect of the Himalayan erosion on the global carbon cycle, which has been debated in the literature (e.g., Raymo and Ruddiman, 1992; France-Lanord and Derry, 1997; Galy et al., 2007; Goddéris and Donnadieu, 2009). Erosion tends to consume atmospheric carbon by two mechanisms. Weathering of silicates produce fluvial alkalinity flux that can later precipitate as carbonate in seawater. Second, plant debris and organic carbon are transported with the particulate flux and can be buried in deep-sea sediment. Ultimately, both mechanisms will take up carbon from the atmosphere and store it over long periods in the sedimentary reservoir. Observation of modern fluxes of erosion as well as of Bengal sediments suggested that the Himalayan erosion mostly consumes atmospheric CO₂ through the burial of organic carbon preferentially to silicate weathering. Nevertheless, it remains impossible to assess the magnitude of these processes at a global scale because the past fluvial fluxes are unknown. The volume and the geochemistry of sediment can provide direct and interpretable records of these fluxes, if their accumulation rates in the Bengal Fan can be determined with sufficient accuracy. Our transect of drill sites at 8°N will document the regional scale of such fluxes during the Neogene that can be extrapolated throughout the entire fan. The deepest penetration site (MBF-3A) will allow exploration of weathering and carbon burial prior to the Neogene.

3. Sampling of the oldest sediment of the fan

Scenarios of the timing and geometry of the collision between India and the rest of Eurasia suggest that collision in the western Himalaya occurred between 50 and 55 Ma and perhaps later (~45 Ma) in the eastern Himalaya, near Everest for instance (Rowley, 1996, 1998). However, there are contrasting models for the slowdown of Indian plate motion and the geometry of the collision (e.g., Dupont-Nivet et al., 2010; Van Hinsbergen et al., 2011; Aitchison et al., 2007; Zhang et al., 2012). When the Himalaya emerged as a mountain range, however, remains less certain. Extending the record of sedimentation back in time should allow a determination not only of when erosion began, but also of when erosion penetrated deep enough into the crust to expose rapidly cooled minerals. In particular, we should be able to determine cooling ages of minerals whose closure temperatures are different, and from the isotopic signature, we should be able to identify what rock of the Himalaya has eroded. Differences between cooling ages and stratigraphic ages will then allow an estimate not only of when erosion began, but also of when erosion exhumed rock from different depths (Copeland and Harrison, 1990; Corrigan and Crowley, 1992; Galy et al., 1996).

Determining when emergence of the Himalaya took place might not provide any surprises. Nevertheless, recall that all of the rock cropping out in the Himalaya was carried by the Indian subcontinent and scraped off it following collision with Eurasia. Thus, given the convergence rate of ~50 km/m.y. that India has moved toward Eurasia since 45 Ma, if collision occurred at 45 Ma but erosion began only at 35 Ma, we might infer that as much as 500 km intact lithosphere was subducted beneath southern Eurasia before a significant mountain range formed. Conversely, if deposition of rock with a Himalayan isotopic fingerprint began shortly after 45 Ma, we must infer that some off-scraping of Indian crust occurred early in the history of the collision to build the initial Himalaya. Finally, we can imagine a flux of sediment early in the history of the collision, but of Tibetan, not Himalayan, origin. This would suggest some, if not necessarily easily quantified, subduction of India beneath southern Tibet before thrust faulting within India created the Himalaya.

The well-known increase in the ⁸⁷Sr/⁸⁶Sr ratio of seawater beginning at ~40 Ma (DePaolo and Ingram, 1985; Hodell et al., 1989) is commonly attributed to increased erosion and weathering in the Himalaya (Edmond, 1992; Galy et al., 1999; Krishnaswami et al., 1992). A strong Himalayan signature, not only beginning at ~40 Ma but also contributing a pulse near 18 Ma (Richter et al., 1992), should, therefore, be corroborated in the sedimentary record of Himalayan erosion. This would be supported if detrital silicates with high ⁸⁷Sr/⁸⁶Sr ratios increased both at ~40 Ma and near 18 Ma. By extending the sedimentary record in the Bay of Bengal through the Oligocene, we can examine the hypothesized correlation of the increased marine Sr isotopic ratio at 18 Ma with weathering of Himalayan rock rich in radiogenic strontium. If we can sample the early history of Himalayan erosion, we can test the assumption that the increasing Sr isotopic ratio beginning at ~38 Ma also results from weathering of Himalayan rock. The sensitivity of the seawater Sr isotopic budget to the Himalayan flux is so high (Galy et al., 1999) that the seawater evolution through time provides a unique system where erosion rate can be estimated using a proxy other than accumulation rates.

The Bengal Fan is one of the thickest sediment sections in the world, and is far too thick for sampling the very old section. The oldest part of the fan sampled to date was at Site 718 more than 2500 km from the present apex of the fan, where early Miocene sediment (~17 Ma) was recovered. Because of its southern position and great water depth, this site may not be adequate to penetrate the “oldest” sediment derived from the Himalaya because the fan may not have prograded so far south (e.g., Curray, 1994). Our deepest proposed site (MBF-3A) is located on the west flank of the Ninetyeast Ridge, ~1300 km from the apex of the fan, where the section is thinner and where a possible Paleocene–Eocene unconformity (“P” horizon) could be reached at a reasonable depth (Curray et al., 1982).

4. Determining fan architecture and spatial depocenter variability

Since the Pliocene, sedimentation in the Bengal Fan has been dominated by deposition in channel-levee systems (Schwenk and Spieß, 2009). During this period, the fan appears to have been built by an accumulation of lenses corresponding to distinct channel-levee episodes intercalated by more slowly accumulating intervals of fine-grained sediment (Fig. F2). Channels carry a flux of sediment for a brief period, apparently only for approximately thousands of years, and then fill with sediment as portions are abandoned and a new channel is cut into the levee system or outside it. Thus, accumulation at any point is likely very irregular, varying from rates >30 m/k.y. for periods as long as 3000 y (Hübscher et al., 1997; Michels et al., 1998; Spiess et al., 1998; Weber et al., 1997) to very low accumulation rates in intervening periods. Nd-Sr isotopic signatures demonstrate that sediment accumulated in levees is dominated by Himalayan material, whereas very low accumulation hemipelagic deposits have distinct isotopic signatures, showing that other sources are mixed with Himalayan flux (Pierson-Wickmann et al., 2001). Determining the distribution and typical lifetime of depocenters is vital for interpreting the older sedimentary record of the fan and assigning different types of sedimentary facies and successions found in deep drill holes to structural units. To address this objective, our six sites are distributed over a 200 km transect designed to obtain sufficient spatial resolution on the basis of a typical width of a channel-levee system on the order of ~50 km. The shallower, ~300 m penetration sites will record the stacking of more than two systems.

Expedition 354 should significantly improve and refine the poor existing age constraints (currently based only on spot-cored DSDP Site 218) of the stratigraphic and structural elements identified in the seismic data. This is especially critical for constraining the transition from early sheet-like turbidite deposition to the onset of channel-levee systems that occurred in the latest Miocene (Schwenk and Spieß, 2009). Because most surface channels reach this part of the fan, it is believed that this marks the start of the development of channel-levee systems on the Bengal Fan generally (Fig. F2). Two reasons might be responsible for the onset of the channel-levee systems: (1) the initial creation of a canyon as point source or (2) changes in the grain size of the delivered sediments transported by turbidity currents to the fan. The three drill sites targeting recovery of late Miocene sediments (MBF-1A, MBF-2A, and MBF-3A) will prove whether there were changes in grain sizes (as interpreted from ODP Leg 116 results) or whether the onset of the channel-levee systems represents the first margin setting with a canyon and probably associated delta. Additionally, because several levees will be penetrated, new insights about the lifetime of distinct channel-levee systems will be gathered, which is so far only known for the active channel (Weber et al., 1997).

Additional objectives

Although Expedition 354 has been designed principally to document Himalayan and Indian monsoon evolutions and interactions and turbiditic fan construction, drilling the Bengal Fan will allow additional important objectives to be addressed. This includes fan hydrology, Bengal Basin deformation, and deep biosphere issues.

Fan hydrology and hydrochemistry

The Bengal Fan is a major sedimentary reservoir filled by continental material, including clays and organic matter that are likely to evolve during diagenesis. Pore water chemistry and O-H isotopic compositions documented on Leg 116 cores revealed a high variability of compositions that imply that low-salinity fluid is released through dehydration reactions probably deeper than in the cored sections and that fluid advection occurs under thermo-convective conditions at least in that portion of the fan (Boulègue and Barriac, 1990; Ormond et al., 1995). There are also indications for diverse diagenetic reactions that release significant amounts of Ca and Sr in the pore water reservoir. Pore water analyses and downhole temperature measurements will constrain the magnitude of compaction, dehydration reactions, and possible fluid advection. This will lead to refined estimates of geochemical fluxes from the fan to the ocean.

Bengal Basin deformation

Throughout the Neogene, the Bengal Basin underwent significant multiphase deformation resulting in the folding of the plate between the Ninetyeast Ridge and the 85°E ridge (Krishna et al., 2001). Beside the Miocene and Eocene unconformities (Curray et al., 2003), the site survey data revealed two additional regional unconformities. Using the DSDP Site 218 age control, these were dated as Pleistocene and Pliocene, respectively (Schwenk and Spieß, 2009). These unconformities are interpreted to be equivalent to unconformities found in the central Indian Ocean, which are related to deformation events of the ocean lithosphere there (Cochran, 1990; Stow et al., 1990; Krishna et al., 2001). Additionally, several faults were identified in the seismic data, especially above the western flank of the Ninetyeast Ridge. These faults terminate within Pleistocene sediments, which also suggest tectonic events at least until the Pleistocene. As Site 218 dating is poorly constrained, we expect Expedition 354 results to allow more precise dating of unconformities and fault terminations, which in turn will improve the understanding of deformation events in the Bengal Basin.

Deep biosphere

Deep biosphere objectives are not specifically planned during this expedition, but the Bengal Fan represents one of the largest fluxes of terrestrial matter to the ocean with relatively fast accumulation. The apparent high preservation of the associated organic matter is a peculiar aspect of Bengal Fan sedimentation (Galy et al., 2007). Microbial activity is intimately intertwined with diagenesis and organic carbon degradation. Because preservation and burial of organic matter is one crucial parameter of the impact of Himalayan erosion on the carbon cycle, there is high interest to study these processes.

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