Clues of Early Life : Dixon Island – Cleaverville Drilling Project ( DXCL-DP ) in the Pilbara Craton of Western Australia Progress Report

In recent years, several scientific drilling programs (e.g.: Archean Biosphere Drilling Project (ABDP), Ohmoto et al., 200�; Deep Time Drilling Project (DTDP), Anbar et al., 2007, Kaufman et al., 2007; PDP: Pilbara Drilling Project, Philippot et al., 2007) were successfully completed in the western Pilbara area, where 3.5, 2.9, 2.7, and 2.5 Ga  sedimentary units were drilled. However, there is a huge time gap in the samples drilled by ABDP and DTDP that represents middle Archean time , between 3.5 Ga and  2.9 Ga (i.e., ~�00 Ma, equivalent to the duration of the entire  Phanerozoic). The Cleaverville-Dixon Island area of the coastal Pilbara terrain (Fig. 1) is suited to filling in the  missing record. It contains well-preserved volcanosedimentary sequences (Cleaverville Group dated at 3.2 Ga) where hydrothermal vein systems, organic-rich siliceous sedimentary rocks, and iron-rich sedimentary rocks are developed (Kiyokawa et al., 200�). Such geological materials may be used to reconstruct past submarine hydrothermal activity and its in��uence on biological activity. Indeed, some attempts have been made to answer the key questions.  However, the surface outcrops in this area are generally weathered to variable degrees; thus they are apparently not  suitable for geo-biological and geochemical studies which require unaltered original chemical/isotopic compositions from the time of their formation in the middle Archean.  Consequently, we carried out the “Dixon Island Cleaverville  Drilling Project (DXCL-DP)”, to obtain “fresh” samples from the sedimentary sequences in the Cleaverville�Dixon Island area.


Introduction
The Pilbara Craton in NW Australia (Fig. 1) exposes one of the well-preserved and least metamorphosed greenstone -preserved and least metamorphosed greenstone preserved and least metamorphosed greenstone belts in the Archean. Greenstone belts are normally composed of a complex amalgam of meta-basaltic and meta-sedimentary rocks. Sedimentary rocks of the greenstone belts are good targets to search for clues of early Earth's environment and life.
In recent years, several scientific drilling programs (e.g.: Archean Biosphere Drilling Project (ABDP), Ohmoto et al.,200�;Deep Time Drilling Project (DTDP), Anbar et al., 2007PDP: Pilbara Drilling Project, Philippot et al., 2007) were successfully completed in the western Pilbara area, where 3.5, 2.9, 2.7, and 2.5 Ga , and 2.5 Ga and 2.5 Ga sedimentary units were drilled. However, there is a huge time gap in the samples drilled by ABDP and DTDP that represents middle Archean time , between 3.5 Ga and Ga and and 2.9 Ga (i.e.,~�00 Ma, equivalent to the duration of the entire ,~�00 Ma, equivalent to the duration of the entire ~�00 Ma, equivalent to the duration of the entire the entire entire Phanerozoic). The Cleaverville-Dixon Island area of the coastal Pilbara terrain (Fig. 1) is suited to filling in the ing in the the missing record. It contains well-preserved volcanosedimentary sequences (Cleaverville Group dated at 3.2 Ga) where hydrothermal vein systems, organic-rich siliceous sedimentary rocks, and iron-rich sedimentary rocks are developed (Kiyokawa et al.,200�). Such geological materials may be used to reconstruct past submarine hydrothermal activity and its in��uence on biological activity. Indeed, some attempts have been made to answer the key questions.
answer the key questions. the key questions. However, the surface outcrops in this area are generally weathered to variable degrees; thus they are apparently not they are apparently not apparently not suitable for geo-biological and geochemical studies which require unaltered original chemical/isotopic compositions from the time of their formation in the middle Archean.
their formation in the middle Archean. formation in the middle Archean. Consequently, we carried out the "Dixon Island -Cleaverville , we carried out the "Dixon Island -Cleaverville the "Dixon Island -Cleaverville "Dixon Island -Cleaverville Drilling Project (DXCL-DP)", to obtain "fresh" samples from the sedimentary sequences in the Cleaverville�Dixon Island area.

Scientific Objectives and Methods
The most important objective of the DXCL-DP is to understand the nature of the middle Archean (3.2 Ga) marine environment in��uenced by hydrothermal activity, through detailed and systematic study of fresh drill core samples.
ed and systematic study of fresh drill core samples. and systematic study of fresh drill core samples. This objective has been pursued through (a) detailed stratigraphy of the whole section, (b) inorganic geochemistry of sedimentary rocks, (c) organic geochemistry of carbonaceous sedimentary rocks (i.e., characterization of the , characterization of the characterization of the carbonaceous materials including insoluble macromolecular matter), (d) study of "microfossils", (e) geochemistry (including stable isotopes) of sulfide in sedimentary rocks, and (f) paleomagnetic study on oriented core samples in order to explore the presence and direction of the geoma-the geoma-geomagnetic field in the early Earth. Various geochemical investigations of shales and cherts are used (e.g., major, minor, trace, and rare earth element geochemistry; C org , N, S, and Fe isotope geochemistry, etc.) to fully extract the information from samples and to understand the in��uence of submarine hydrothermal activity on the biological and chemical fingerprints. From these data we intend to determine the original determine the original the original environmental information at the time of deposition.

Drilling Targets and Geological Background
Our drilling targets were deeply buried organic-rich black ly buried organic-rich black buried organic-rich black cherts and black shales of the Dixon Island Formation (DIF) and Snapper Beach Formation (SBF) (Fig. 2). Both formations belong to the 3.2 Ga Cleaverville Group and are exposed along the coasts of the Cleaverville area and Dixon Island.
the Cleaverville area and Dixon Island. Cleaverville area and Dixon Island. The Cleaverville Group is a well-preserved submarine sequence affected only by low-grade metamorphism (prehnite-pumpellyite facies) without intensive deformation

Technical Developments
Clues of Early Life: Dixon Island-Cleaverville Drilling Project (DXCL-DP) in the Pilbara Craton of Western Australia (Kiyokawa et al., 2002). It is composed of volcanic rock units It is composed of volcanic rock units is composed of volcanic rock units and chemical-volcano-sedimentary sequences. Interpretation of its depositional settings diverges among rift, horizontal tectonics, accreted oceanic crust, or accreted immature island arc (Hickman, 1983;Van Kranendonk et al., 200�).
The ~350-m-thick DIF in the lower part of the Cleaverville -m-thick DIF in the lower part of the Cleaverville m-thick DIF in the lower part of the Cleaverville -thick DIF in the lower part of the Cleaverville thick DIF in the lower part of the Cleaverville Group consists mainly of highly silicified volcanic-siliceous sequences that contain apparent microbial mats and fossilized bacteria-like structures within black chert and bacteria-like structures within black chert and -like structures within black chert and like structures within black chert and ike structures within black chert and structures within black chert and structures within black chert and also includes a komatiite-rhyolite sequence bearing hydrothermal veins. The >300-m-thick SBF in the upper part of the -m-thick SBF in the upper part of the m-thick SBF in the upper part of the -thick SBF in the upper part of the thick SBF in the upper part of the Cleaverville Group contains a thick unit of reddish shale, bedded red-white cherts, and a banded iron formation. It also , and a banded iron formation. It also and a banded iron formation. It also also contains chert fragments bearing pyroclastic beds bearing bearing pyroclastic beds bearing bearing pyroclastic beds bearing chert fragments (Kiyokawa et al.,200�). We selected two coastal sites at the eastern part of the Cleaverville Beach for drilling (Fig. 3). The first site (CL1 and CL2) was intended to The first site (CL1 and CL2) was intended to irst site (CL1 and CL2) was intended to (CL1 and CL2) was intended to CL1 and CL2) was intended to ) was intended to to drill through the lower part of the SBF (distance between the lower part of the SBF (distance between lower part of the SBF (distance between holes is �0 m along the core dip direction), and the other is m along the core dip direction), and the other is m along the core dip direction), and the other is the other is other is the DX site which was targeted to DX site which was targeted to which was targeted to to drill the upper DIF.

Drilling Results
DXCL-DP was successfully completed in summer 2007 after after continuous drilling from 31 July to drilling from 31 July to from 31 July to 31 July to July to 10 August. The orientation of the August. The orientation of the core, being perpendicular to the bedding plane, is 52º dip to the southwest for the CL1 and CL2 site Cleaverville Group, Pilbara Supergroup and 52º dip to the northwest for the DX site. Orientations of the core samples were taken using "Ezy-Mark" oriented system (2iC Australia Pty Ltd). As a cooling media during drilling, freshwater was used at CL1 and CL2 sites, and seawater at DX site; for both, partially hydrolyzed polyacrylamide lubricant was added. A summary of information on drilling sites (core length, direction, bedding, etc.) is presented in Table 1. Stratigraphic columns of CL1, CL2, and DX are shown in Fig. 4, and an example of drillcore (DX) in a . 4, and an example of drillcore (DX) in a 4, and an example of drillcore (DX) in a , and an example of drillcore (DX) in a n example of drillcore (DX) in a core tray is shown in Fig. 5.
. 5. 5.   yellowish-brown rock (WY), black shale (BS), and reddish -brown rock (WY), black shale (BS), and reddish brown rock (WY), black shale (BS), and reddish , and reddish and reddish shale (R) units. Boundaries between each units are highly fragmented. The WY unit, containing laminated white chert, is highly weathered. The BS unit was subdivided into five subunits, BS1 to BS5. Each unit mainly consists of massive BS5. Each unit mainly consists of massive 5. Each unit mainly consists of massive black shale with well-laminated black shale and silt bed, and contains some fine sandstone with cross lamination. The R unit was also subdivided into five subunits (R1 to R5), which R5), which 5), which , which which which consist of reddish massive shale, white chert, and massive and well-laminated black-gray-red shale. The color changes between red and brown are gradual. The CL2 drillcore is rown are gradual. The CL2 drillcore is own are gradual. The CL2 drillcore is generally more fragmented than CL1 and DX drillcores. Bedding orientation slightly changes at 75-7� m depth and 81-82 m depth.
DX. . The DX drill core (100.2 m long; Fig. 4), covering the m long; Fig. 4), covering the m long; Fig. 4), covering the upper part of DIF, consists of four units: highly fragmented/ deformed and well-laminated black shale (�9-88 m, with disturbance at 85-88 m); well-laminated black shale with pyrite lamina (88-149 m; a partly yet highly deformed section a partly yet highly deformed section partly yet highly deformed section yet highly deformed section highly deformed section at 101-110 m; see  deformed/fragmented zones. The normal dip of the DX drillcore is approximately 50º. Gradual changes in the dip orientation are observed at the 110-123 m depth range that 110-123 m depth range that 110-123 m depth range that exhibit a few meter-scale open folds. The uppermost ~70 m of the drillcore DX is generally highly weathered. Its upper part (47.9-59 m) is massive and reddish, the middle part (59-�3 m) contains bleached materials, and the lower part (�3-�9 m) is red but preserves the chilled margin structure of pillow chilled margin structure of pillow chilled margin structure of pillow basalts.

Summary
"Dixon Island-Cleaverville Drilling Project (DXCL-DP)" was successfully completed in summer 2007. Three holes were cored, the CL1 and CL2 cover the Snapper Beach Formation, while the DX drillcore covers the Dixon Island Formation, both of which belong to the Cleaverville Group. The CL1 and CL2 drillcores consist mainly of organic-rich massive black shales with little cross-laminated fine sandstone, and the DX drillcore contains very finely laminated black shales with lamination and veins of pyrite and weathered pillow basalt. These sulfide-containing black shales are not found anywhere in surface outcrops. It is the first discovery of these geological units.
A systematic combination of geological, sedimentological, geochemical, and geobiological approaches to the drillcore s to the drillcore to the drillcore samples will be applied to obtain critical information on the characteristics of the samples and to understand the in��uence of submarine hydrothermal activity on the biological and chemical fingerprints. From these we intend to reconstruct . From these we intend to reconstruct From these we intend to reconstruct From these we intend to reconstruct rom these we intend to reconstruct these we intend to reconstruct we intend to reconstruct the environmental conditions at the time of deposition.
conditions at the time of deposition. at the time of deposition.