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Expedition-specific challenges, risks, and future perspectives

Quality assurance/Quality control

The riser drilling technology and the associated use of drilling mud had considerable impact on our scientific program. Whereas contamination control has become an integral measure of quality assurance in riserless ODP/IODP expeditions with focus on subseafloor life (Smith et al., 2000a, 2000b; House et al., 2003; Lever et al., 2006), the riser drilling procedure required a more rigorous QA/QC program during Expedition 337. Most severely affected by the contamination risk were the scientific objectives of the fluid chemistry and microbiology disciplines. Therefore, our sampling programs of both fluid chemistry and microbiology included routine monitoring of the integrity of samples selected for interstitial water and microbiology analysis by X-ray CT scanning (see Fig. F16). Heavily disturbed or fractured samples were returned into the normal core flow before processing, and alternative samples with lower risk of contamination were selected instead. An example of one such heavily disturbed, and thus contaminated, sediment interval is shown in Figure F17. Because drilling mud was highly divergent in chemical composition from sediment pore water, even minor contamination would complicate the accurate analysis of routinely measured interstitial water parameters such as sulfate, pH, or alkalinity, all of which are directly relevant to the study of deep subseafloor life. Moreover, microbial cell concentrations in drilling mud were consistently >108 cells/cm3. This is orders of magnitude higher than pelagic seawater used during past riserless drilling expeditions (Lever et al., 2006) and two to five orders of magnitude higher than expected concentrations of indigenous cells in sediment deeper than 1000 mbsf (Parkes et al., 2000), and thus increases the risk of contamination with nonindigenous cells substantially compared to previous riserless drilling operations.

The use of chemical and microbial contamination tracers was pioneered for riser scientific ocean drilling as part of an extensive QA/QC program. Contamination monitoring with perfluorocarbon (PFC) compounds as chemical tracers utilized a modified version of past protocols (Smith et al., 2000a, 2000b; House et al., 2003; Lever et al., 2006). PFC tracer was added daily to drilling mud tanks. Detailed sampling and analyses of drilling mud, sediment cuttings, and core samples provided valuable quantitative estimates of the volume of drilling mud and number of cells introduced into samples during riser drilling (Fig. F16). PFC concentrations monitored within drilling mud in tanks, core liners, and the mud ditch after recovery show consistently high values and low loss during drilling operations (mean concentration > 100 µg/L). PFC measurements within cores show a wide range of values, with high contamination near the core liner (exterior; typically 1–100 µL drilling fluid/g sediment) and low values in the core center (interior; 0.01–1 µL/g). DNA-based contamination tests targeting organisms associated with surface seawater, drilling mud viscosifiers, and sewage reveal drilling mud viscosifiers as the main source of drilling-induced microbial/DNA contamination.

Data obtained during Expedition 337 demonstrate the suitability of PFC tracers to monitor contamination during riser drilling operations and indicate that many core samples obtained have low to nondetectable levels of contamination at the core center. The successful detection of cells and DNA demonstrates that monitoring of microbial populations in cores obtained by riser drilling is possible on board the ship. Shore-based molecular analyses and cultivation experiments in the coming years will reveal the extent to which cells and DNA detected represent indigenous microbial communities, and, if so, what the metabolism of these microbes is. Cultivation assays will be carefully monitored for marker genes of potential contaminants and designed to specifically select for deep subseafloor rather than contaminant microbial populations (Fig. F15).

During fluid sampling and analyses of inorganic constituents, we sought to minimize contamination by relying on prior information of X-ray CT scan images and by thoroughly peeling off outer layers that were in contact with the core liner and thus drilling mud and avoiding fractures as much as possible. Nevertheless, residual levels of contamination were not avoidable, as illustrated in Figure F18. The fraction of drilling mud contamination in the total interstitial water was calculated assuming binary mixing of several major ions that are present in much greater concentrations in the drilling mud than in interstitial water. Likewise, we assumed that the degree of natural variation of these ions in the interstitial water composition was small compared to variation caused by contamination. The most contaminated samples were typically sandy mudstone at the top of Unit II. Results were variable based on the ion chosen but provide some indication of the degree of contamination. For example, the mud-derived water fraction of pore water in the most severely contaminated, shallowest whole-round sample (337-C0020A-1R-2, 0–65 cm; sandy; 1278 m CSF-B) varies from 65% to 88% depending on whether salinity, K+, SO42–, or Cl are considered as the interstitial water end-members in binary mixing. Consequently, corrections of other interstitial water constituents for their dilution with drilling mud is associated with inherent, residual uncertainties. For the majority of interstitial water samples obtained toward the bottom of Unit II and deeper, contamination is significantly lower. In these horizons, we estimate a range of contamination from 2% to 20%.

Considerable challenges were also posed on organic geochemical and micropaleontological analyses by contamination from drilling mud. TOC concentrations of cuttings were suspiciously elevated relative to core samples from nearby horizons. Lipid extracts of cuttings and some core samples showed molecular signatures of the asphalt-based drilling additive ASTEX-S (trade name; also known as sulfonated asphalt sodium salt [SAS]); cuttings in shallower horizons of Hole C0020A showed other unidentified, highly concentrated contaminants that probably result from the drilling technology (Fig. F19). Moreover, micropaleontological analyses of diatom assemblages in core samples, core catchers, and especially cutting samples were affected by a “memory effect.” For example, relatively young diatom assemblages representative of shallower late Miocene and Pliocene strata were abundant in drilling mud and complicated the use of cuttings for diatom-based chronostratigraphy. Focus on undisturbed core samples eliminated this problem but also substantially lowered the number of samples and thus the depth resolution that could be examined for age-relevant marker fossils.

Preliminary evidence for deep subseafloor microbial activity associated with coalbeds

The main goals of Expedition 337 were related to microbial life associated with deeply buried coalbeds. In order to tackle this set of goals within the Shimokita coalbeds, we had to drill deeper than any previous expedition of scientific ocean drilling. Both microbial activity and cellular concentrations are expected to decrease with sediment depth and age (e.g., Parkes et al., 2000; Jørgensen, 2012). Even if our samples were uncontaminated, the detection and examination of such deep subseafloor microbial life would not be trivial and would require both state-of-the-art methodology and utmost scientific scrutiny. With the added difficulty of contamination, we will need to build a case that integrates various lines of biological and chemical evidence for the presence and activity of microbes and the nature of microbially mediated processes.

This means it is necessary to assemble preliminary lines of evidence that are suggestive of microbial life associated with the coalbed and provide first answers to some of the expedition objectives. The most compelling evidence for microbially mediated methanogenesis is found in our gas compositional data. In particular, C1/C2 ratios (Fig. F12) analyzed during mud-gas monitoring are generally in the range suggesting biological methanogenesis as the major source of methane (e.g., Whiticar, 1999). The most striking result, however, is the strong positive inflection of this ratio associated with the major coal-bearing horizons at Site C0020 (i.e., Unit III at ~2 km subseafloor depth); this trend to higher values is consistent with coalbeds being an active source of methane to the deep formation. This interpretation of an important role of methanogenesis is supported by stable carbon isotopic compositions of methane that have been recorded in real-time mud-gas monitoring (data not shown). Further validation of isotopic relationships of different carbon pools and determination of stable hydrogen isotopic compositions of methane will provide more detailed information regarding the pathways and substrates utilized by methanogens.

Although further validation in shore-based laboratories is required, other lines of evidence such as extracted DNA or visual observation of intact cells are consistent with the presence of indigenous microbial populations at great burial depth at Site C0020. The sole detection of functional genes indicative of methane cycling in sediment samples (and not in drilling mud) is promising. Moreover, intact cells were detected in deep horizons (Fig. F14) and show generally very low cellular abundances. These very low concentrations in carefully cleaned samples are on the one hand encouraging as they suggest that contamination has not resulted in vastly elevated cellular counts; on the other hand we are faced with the relatively highest potential impact of contamination when in situ cell densities are already low. Additional molecular work on the single cell level to the system level is thus required to confidently assign these cells to indigenous populations.

We also have obtained the first indication related to the activity of the subseafloor biosphere in deep horizons associated with the coalbed. Whereas the compositions and concentrations of various gases suggest a stimulation of microbial activity through the coalbed, the relatively low levels of alkalinity suggest substantially lower rates of microbially mediated remineralization of carbon than in the upper few hundred meters of previously studied deep biosphere sites (e.g., Shipboard Scientific Party, 2003; Expedition 311 Scientists, 2006), including shallower sediment at this location (Aoike, 2007; Tomaru et al., 2009). Nevertheless, there is roughly a two-fold elevation in alkalinity within the coal-bearing Unit III, relative to the overlying Unit II. This supports other lines of evidence suggestive of stimulation of microbial activity through the coalbed. In conclusion, the coalbed at ~2 km subseafloor depth is probably not directly responsible for the presence of methane hydrates found in shallower layers at this site (Aoike, 2007); instead, it resembles a slow-paced bioreactor with sustained activity on geologic timescales as previously proposed for other organic-rich deeply buried layers such as Cretaceous black shales at Demerara Rise (Arndt et al., 2006).

Preliminary assessment of the sedimentation history at Site C0020

Our sedimentary analysis of the depositional environment, revealed through Hole C0020A, is that >700 m of intertidal and wetland sequences were deposited from late Oligocene/early Miocene through early/middle Miocene. This result is inconsistent with our expectations that such young sequences would not significantly thicken and that Eocene lignite layers would be present under an Oligocene unconformity layer. Our findings suggest that this sedimentary basin had been continuously subsiding in order to generate the accommodation space during this period without an abrupt faulting event and that the rate of basin subsidence had been in balance with the sedimentary input. For a better understanding of such basin dynamics and formation mechanisms, further investigation of the regional geology and tectonics is required.

Accomplishments and future perspectives

During Expedition 337, our major operational objectives (Inagaki et al., 2010) were successfully accomplished through use of the riser drilling system of the Chikyu. The bottom depth of Hole C0020A is 2466 mbsf, extending the previous maximum penetration depth in scientific ocean drilling by 355 m and providing the chance that our postcruise research will extend the widely accepted evidence of deepest subseafloor life by up to 800 m. The cored materials provide unprecedented opportunity to address fundamental scientific questions pertaining to the interactions between a deep coalbed hydrocarbon system and subseafloor life. New shipboard facilities on the Chikyu such as the mud-gas monitoring laboratory and the radioisotope laboratory were successfully implemented and strongly contributed to the mission achievement of Expedition 337. The core recovery through riser drilling was remarkably high (75.3% in average), often close to 100% (12 out of 32 spot cores were >90% recovery), even at great burial depths of 2000 mbsf and deeper. The cored materials include diverse lithologies (e.g., lignite coal, sandstone, silty mudstone, beach sand, carbonate minerals, and conglomerate). Extremely soft sand layers (i.e., beach sands), which had been almost impossible to retrieve in intact form by riserless drilling, were successfully recovered by riser drilling using high-density mud. The condition of the riser borehole was excellent, allowing close-to-perfect acquisition of downhole wireline logging data. The successful accomplishment of the aforementioned tasks required the technological capabilities of the riser drilling vessel Chikyu. Therefore, Expedition 337 has been an important step into a new era of scientific ocean drilling in which Earth and life scientists will jointly explore the deep realms of our planet that have never been studied before.

This first deep riser drilling expedition exploring deep life also had important strategic value in that this was the first time that the impact of commercially used drilling technology was rigorously tested by a large team of biologists, chemists, and geologists for its compatibility with the scientific goals. As a result, a number of recommendations related to the future use of this technology in scientific ocean drilling can be made. These relate to (1) coring technology, (2) drilling mud composition and sterilization, and (3) the use of deep riser holes for experiments:

  1. During Expedition 337, we performed spot coring, instead of a conventional sequential coring strategy, using standard 8½ inch rotary core barrel (RCB) coring and 10⅝ inch industry-type large diameter coring (LDC) system. Both coring systems resulted in excellent quality cores, including very hard carbonate-cemented nodules and conglomerates, from scientifically important horizons. This spot coring strategy is essential for reducing the cost and time for riser drilling operation. LDC cores maximize the probability of obtaining uncontaminated massive core samples that are adequate for high recovery of pore water, allowing highly sensitive and specific biogeochemical and microbiological analyses. However, the use of an aluminum core liner required modification of the normal workflow and resulted in much longer time requirements for delivery of core material from the rig floor to the laboratory. Nevertheless, we processed LDC cores under anaerobic conditions and retrieved useful data and samples with relatively low levels of contamination for shipboard and shore-based analyses. Considering the high risk of drilling mud and microbial contamination of the standard RCB core, it would be desirable to explore the potential use of improved LDC-type coring systems with nonmetal core liner (e.g., carbon glass fiber or reinforced plastic liner) as the standard spot-coring tool for future deep scientific drilling on the Chikyu.

  2. The use of riser drilling mud is essential for future deep scientific explorations. On the other hand, we need to address the causes of contamination to enhance detection of indigenous signatures of life and geochemical characteristics. The mud used during Expedition 337 contained ~108 contaminated cells per 1 cm3, even though the fluid is alkaline and contains sterilizing chemicals. This high concentration of nonindigenous cells complicated precise detection of deep microbial life and its metabolic activities and influenced the chemical composition of pore water. To minimize the risk of drilling mud–related sample contamination during future scientific riser drilling expeditions, alternative drilling mud compositions should be considered. For example, are there feasible technologies for mud sterilization that could be implemented without conflicting with operational demands for deep drilling? Can the organic additives that appear to nourish microbial communities be substituted with inorganic components (e.g., silicon)? Can we develop in situ sampling devices for recovering noncontaminated and biologically pristine core and fluid samples?

  3. A positive aspect of deep-riser drilling is the superior borehole stability supported by the use of high-viscosity mud that prevents possible collapse and flow down of rubbly horizons such as coal and fault layers. This is not only useful for coring materials with high recovery rate, but also essential for successful completion of multiple deployments of logging tools, including downhole in situ fluid sampling and analysis. With the combined use of borehole observatory sensors and subseafloor laboratory equipment, the maintenance of stable deep-riser boreholes will be highly useful for advanced subseafloor research in short- to long-term projects.

Last but not least, this expedition also provided a testbed for the use of riser drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling. Since the riser system was originally developed by the petroleum industry, the Chikyu is equipped with a mature technology. However, the adaptation of this technology to the needs of basic science will be an important challenge that needs to be addressed as an integral component in plans for the next riser missions. Implementation of science-oriented deep-riser drilling in IODP would provide grand opportunities for Earth system sciences.