Data Report: Carbon and Oxygen Isotope Geochemistry along a Subducting Pelagic Section offshore Costa Rica (ODP Legs 170 and 205)

Abstract

In this data report we present results from stable isotope measurements (δ13C and δ18O) on bulk sediment at several sites located on a transect along a subduction margin offshore Costa Rica (Ocean Drilling Program Sites 1039, 1040, and 1253). Comparison of stable isotope compositions (δ13C and δ18O) of the pelagic carbonates Subunit U3C between the reference sites (Site 1039 and 1253) and the underthrust section (Site 1040) reveals similar δ13C values and minor differences in δ18O values within four specific intervals. Isotope stratigraphy was then used to further constrain the shipboard age models based on bioand magnetostratigraphy. The resulting age models are in agreement with those derived from biostratigraphy and confirm that the sedimentation rate of the lower Subunit 3C is roughly constant on the order of 50 m/ m.y. This is in contrast with the postulated very high sedimentation rates at ~12.7 Ma and lower sedimentation rates (~18 m/m.y.) in the lower part of the section between 16 and 13 Ma, as suggested by shipboard magnetostratigraphic datums. 1Strasser, M., Weissert, H., and Bernasconi, S.M., 2006. Data report: Carbon and oxygen isotope geochemistry along a subducting pelagic section offshore Costa Rica (ODP Legs 170 and 205). In Morris, J.D., Villinger, H.W., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 205, 1–18 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Geological Institute, ETH Zurich, Universitaetsstrasse 16, CHN H71, CH8092 Zurich, Switzerland. Correspondence author: strasser@erdw.ethz.ch Initial receipt: 28 February 2005 Acceptance: 22 November 2005 Web publication: 3 April 2006 Ms 205SR-205 M. STRASSER ET AL. DATA REPORT: CARBON AND OXYGEN ISOTOPE GEOCHEMISTRY 2 INTRODUCTION The Cocos plate offshore Costa Rica comprises igneous oceanic crust covered by 450 to 500 m of pelagic deposits. Along the Costa Rica convergent margin, this plate is being subducted beneath a prism wedge attached to the Caribbean plate (Morris, Villinger, Klaus, et al., 2003, and references therein) (Figs. F1, F2). During Ocean Drilling Program (ODP) Legs 170 and 205, the sedimentary succession of the subducting plate was drilled at reference sites oceanward of the deformation front (Sites 1039 and 1253) and was also recovered at a prism site (Site 1040) below the decollement (Fig. F2). At reference Site 1039, three sedimentary units and one intrusive unit were recovered (Kimura, Silver, Blum, et al., 1997) (Fig. F3). Unit U1 consists of dark olive-green diatomaceous ooze intercalated with ash layers. Below a sharp contact, Unit U2 is distinguished by a rapid decrease in biogenic sediment and consists of dark olive-green silty clay interbedded with light olive-green calcareous clay and ash layers. Unit U3 exhibits a dramatic increase in biogenic sedimentation, changing sharply from the nearly barren clays of Unit U2 to ivory to light green and mottled siliceous calcareous oozes interbedded with calcareous clay and ash. The basal oozes of Subunit U3C are metaliferous. The igneous Unit U4 consists of fine-grained gabbros that are interpreted as intrusive sill (Kimura, Silver, Blum, et al., 1997). Below this ~35-m-thick igneous unit, another ~20 m of pelagic sediment (Subunit U3C) was recovered during Leg 205 at Site 1253 (Morris, Villinger, Klaus, et al., 2003). Drilling at Sites 1040 and 1043 and subsequent studies (Morris, Villinger, Klaus, et al., 2003, and references therein) show that the sediment section beneath the decollement repeats the complete lithology and sequence of the subducting plate cored at Site 1039 (Fig. F3). Also, the seismic observations indicate complete sediment subduction past the prism front (Fig. F2). The thinning of the underthrust section seen between Sites 1039 and 1040 (Figs. F2, F3) must then reflect compaction and dewatering processes. This leads to a change of the corresponding lithologies from silty clay and calcareous oozes at Sites 1039 and 1253 to claystones and calcareous chalk in the underthrust section at Site 1040. The progressive subduction of the pelagic sediment section along the Costa Rica convergent margin offers the possibility to study in detail compaction and dewatering processes during early subduction. The signature of such processes should be seen in chemical proxies due to diagenesis and alteration of the pelagic sediments. By comparing the chemical signature in the sediments at the reference sites with those of the underthrust section, effects of compaction and dewatering processes on diagenesis could be estimated. In this data report, we present results on stable isotope compositions that show significant differences in the δ18O values between the unaffected carbonates in sedimentary Unit U3 at reference Sites 1039 and 1253 and the equivalent underthrust section at Site 1040. These results could be used in further studies to test models of sediment compaction and dewatering processes during early subduction. In addition, the stable isotope analyses of the pelagic carbonates in the reference site oceanward of the deformation front (Site 1039) are used as a chemostratigraphic tool to complement the bioand magnetostratigraphy of the carbonate section (Kimura, Silver, Blum, et al., -8.5 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 E st P aific R se Cocos plate Caribbean plate Nazca plate Galapagos Islands Co co s R idg e Carnegie Ridge South America osta ica iddle erica rench