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Ocean Drilling Program Scientific Results Volume Haggerty, J.A., Premoli Silva, I., Rack, F., and McNutt, M.K. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 144 43. CARBONATE AND PORE-WATER GEOCHEMISTRY OF PELAGIC CAPS AT LIMALOK AND LO-EN GUYOTS, WESTERN PACIFIC1 Carsten Israelson,2 Bjorn Buchardt,2 Janet A. Haggerty,3 and Paul N. Pearson4 ABSTRACT Pelagic sediments accumulated on Limalok and Lo-En guyots as they sank below the wave base. Limalok Guyot contains 140 m of pelagic sediments that range in age from the early Miocene to the Holocene. Lo-En Guyot contains approximately 150 m of pelagic sediments of late Oligocene to Holocene age. Ten different geochemical parameters were measured in an attempt to evaluate the potential use of the sediments as a geochemical recorder of the environment. It was found that, although the pelagic caps consist of well-preserved planktonic foraminifers, mixing of sediments during or shortly after deposition precludes high- resolution paleoceanographic studies. The original pore-water composition is modified by interactions with the sediment and possibly by seawater at a postdepositional time. INTRODUCTION SITE SUMMARIES AND SEDIMENTS Background The sites studied are located on Limalok (Site 871) and Lo-En (Site The purpose of the present study is to evaluate pelagic caps on 872) guyots in the western Pacific Ocean (see site map preceding the western Pacific guyots as geochemical recorders and to assess their title page). Pelagic sediments were recovered in all three cases using suitability for paleoceanographic studies. the advanced hydraulic piston corer (APC). In general, the foraminifer The guyots studied are Limalok and Lo-En guyots, which belong or nannofossil foraminifer ooze was soupy upon recovery. This car- to the Marshall Island group of seamounts. Pelagic sediments accu- bonate ooze had a very high water content and contained only a small mulated on these guyots as they slowly sank below the wave base percentage of clay, unlike the majority of pelagic carbonate sequences after the drowning of the carbonate platform. This means that the drilled by the Deep Sea Drilling Project (DSDP) or ODP. Liquefaction pelagic sediments were deposited well above the carbonate compen- appeared to occur where the sediments were disturbed, although some sation depth (CCD) and consist almost solely of planktonic foramin- sedimentary structures were recognized in the cores (Premoli Silva, ifers and nannofossil ooze with a very low clay content. Haggerty, Rack, et al., 1993). Downhole contamination caused by caving from the top of the hole was a problem for biostratigraphic age Four kinds of geochemical parameters were studied in these sedi- assignment in several cases (Pearson, this volume). ment samples: carbonate concentration, element ratios (Sr/Ca, Mg/ Ca), stable isotopes (δ18θ and δ13C), and radiogenic isotopes (87Sr/ 86Sr). Furthermore, four kinds of parameters were studied in the pore Limalok Guyot (Site 871) water samples: element concentration, stable isotopes (δ'8O and δ2H), radiogenic isotopes (87Sr/86Sr), and short-chain organic acids. Limalok Guyot is located toward the southern end of the eastern Ratak Chain of seamounts (see site map preceding the title page) and is the best studied of the two guyots described herein. Site 871 Previous Studies (5°33.4'N, 172°20.7'E) was drilled through the central and thickest Legs 143 and 144 are the first Ocean Drilling Program (ODP) part of the pelagic cap at a water depth of 1255 m. Of the two holes expeditions to drill through all of the lithologies of Pacific guyots, cored (871A and 871B), the most complete recovery of pelagic sedi- including the volcanic pedestal. Only Leg 144 recovered pelagic ment (90.7%) was obtained in Hole 871 A, which also provided all of cap sediment; therefore, literature on the geochemical properties of the samples from Site 871 analyzed in this study. Refusal occurred at pelagic caps is sparse. Sedimentation and erosion processes on nearby 139.5 meters below seafloor (mbsf) as the piston core reached a black, Horizon Guyot have been described by Lonsdale et al. (1972). As laminated iron-manganese crust. The manganese crust overlies an with the guyots studied in this paper, Horizon Guyot has a pelagic cap Eocene limestone platform. that consists of Tertiary nannofossil and planktonic foraminifer ooze. The pelagic sediments accumulated at Limalok Guyot (Site 871) Lonsdale et al. (1972) found that the pelagic sediment had suffered can be divided into two subunits (Premoli Silva, Haggerty, Rack, et al., continuing erosion that had winnowed away the fine components of 1993). Subunit IA covers the interval from 0 to 25.1 mbsf. This unit the sediment. Bed load transport of the remaining foraminiferal sand comprises homogeneous nannofossil foraminifer ooze. Age assign- created regular trains of ripple and dunes. ments from both planktonic foraminifers and nannofossils indicate a Pleistocene to upper Pliocene biostratigraphic age (Premoli Silva, Haggerty, Rack, et al., 1993; Pearson, this volume). Subunit IB ranges from 25.1 to 139.5 mbsf and consists of homogeneous foraminifer Haggerty, J.A., Premoli Silva, I., Rack, R, and McNutt, M.K. (Eds.), 1995. Proc. ooze with a winnowed, well-sorted, medium-sized sand texture. The ODP, Sci. Results, 144: College Station, TX (Ocean Drilling Program). 2 unit is dominated by planktonic foraminifers (95%); nannofossils Geological Institute, University of Copenhagen, 0ster Vbldgade 10, DK-1350 Copenhagen K, Denmark. make up less than 5% of the unit. Biostratigraphic dating of Subunit IB 3 Department of Geosciences, University of Tulsa, 600 South College Avenue, Tulsa, indicates an age of early to middle Miocene (Pearson, this volume). OK 74104, U.S.A. 4 However, micropaleontological and Sr-isotope studies have demon- Department of Earth Sciences, University of Cambridge, Downing Street, Cam- strated that sedimentation was episodic and reworking was common bridge, CB2 3EQ, United Kingdom. (Present address: Department of Geology, Wills Memorial Building, University of Bristol, Queens Road, Bristol BS8 1RJ, United (Israelson et al. and Pearson, both in this volume). The lower Miocene Kingdom.) interval was particularly unsuited for biostratigraphic dating, not only 737 C. ISRAELSON ET AL. because of upward sediment reworking of zonal marker species, but bridge. For information on technical procedures for single-species for- also because of the lack of well-defined zone fossils (Pearson, this aminifer analyses, see Pearson and Shackleton (this volume). For the volume). It was not possible to establish a detailed foraminifer zona- bulk-sample analyses, approximately 20 mg of sediment was used. tion below 80.5 mbsf (Cores 144-871A-9H to -15H). Every sample was analyzed twice, as duplicate runs included complete chemical processing of each sample. For 13C/12C and 18O/16O determi- Lo-En Guyot (Site 872) nations, a Finnigan MAT 250 triple-collector mass spectrometer was used. Sample compositions were measured against the laboratory stan- Site 872 (10°05.6'N, 162°52.0'E) was drilled in the central and dard (LEO) and were calculated as PDB values (δ18θ). Precision for thickest portion of Lo-En Guyot at a water depth of 1084 m. Of the oxygen and carbon isotope determinations, expressed as ± standard two adjacent holes (872A and 872C), Hole 872C was most complete, deviation for 10 preparations of standard carbonate, is better than with 97.7% recovery of pelagic sediment. Therefore, all analyzed ±0.07%o on the delta scale for both oxygen and carbon isotopes. samples, are from this hole. APC refusal occurred at 141.7 mbsf, at the boundary between upper Oligocene pelagic sediments and altered Sr Isotopes basalt. The contact between altered basalt and Oligocene sediment consists of partly lithified fragments of white foraminifer ooze. Only single species of planktonic foraminifers were used for the The pelagic sediments at Lo-En Guyot have been divided into two Sr-isotope analyses presented in this study. Isotope ratios were mea- subunits based on the downhole decrease in nannofossil abundance sured in the dynamic mode on a VG Sector 54 thermal ionization mass and a corresponding coarsening of the sediment texture (Premoli Silva, spectrometer at the Danish Center for Isotope Geology, University of Haggerty, Rack, et al., 1993). Subunit IA (interval from 0 to 32.9 mbsf) Copenhagen. The average value of the NBS-987 standard, measured 87 86 consists of foraminifer ooze and is interbedded with subordinate, pale throughout the 6-month period of the Leg 144 study was Sr/ Sr = 6 brown nannofossil foraminifer ooze in vague layers, which are several 0.710257, with a standard deviation of ±14.8 × 10" (N = 45). The centimeters to tens of centimeters thick. Foraminifer biostratigraphic error quoted for all samples in this study is the average difference 6 data suggest a Pleistocene-Pliocene age for Subunit IA. Age determi- between sample duplicates, which is 19.8 × 10~ . For more informa- nation based on planktonic foraminifers was problematic because of tion on these analytical procedures, see Israelson et al. (this volume). the severe fragmentation of the foraminifer tests (Pearson, this vol- ume), although nannofossils provided detailed biostratigraphic zona- Organic Geochemistry tion (Watkins, this volume). Subunit IB covers the interval from 32.9 to 141.7 mbsf and consists of soupy homogeneous foraminifer ooze The preservation of organic matter in the pelagic sediment was with less than 5% nannofossils. This interval
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