*To whom correspondence should be addressed: FAX: (886) ;

Transcription

1 DATA REPORT: CARBONATE AND ORGANIC CARBON CONTENTS OF SEDIMENTS FROM SITES 1143 AND 1146 IN THE SOUTH CHINA SEA Li-Wen Wang* and Hui-Ling Lin Institute of Marine Geology and Chemistry National Sun Yat-Sen University Kaohsiung, Taiwan 804 *To whom correspondence should be addressed: FAX: (886) ; For submission to: Ocean Drilling Program Scientific Results, Volume 184

2 INTRODUCTION The goals of Ocean Drilling Program (ODP) Leg 184 were to reconstruct the long-term paleoenvironmental variability of the South China Sea (SCS) and the adjacent Asia continent. In the paleoceanographic proxies, the amounts of inorganic and organic carbon that have accumulated in sediment are fundamental measurements of paleo-productivity (e.g., Meyers, 1997). These proxy indicators are mainly controlled by surface water productivity and/or sediment preservation. In this report, we have augmented the shipboard data with coarse fraction contents (weight percentage of particles > 63 µm) from Sites 1143 and In addition, we compared the data from these two sites to provide a contrast of the paleo-environment for the southern and northern parts of the SCS. Site 1143 ( N, E) is located within the Nansha area on the southern margin of the SCS with a water depth of 2772 m, recording the depositional history of 9 Myr. Site 1146 ( N, E) is at a water depth of 2092 m on the mid continental slope of the northern part of the SCS, covering a time interval of 18 Myr. Both sites lay above the modern lysocline (~3000 m). Sediment samples were collected at a frequency of one per section meter from three APC/XCB holes of the two sites, including those squeeze cakes (Table 1). METHODS Subsamples were divided for coarse-fraction and geochemical analyses. For coarse-fraction, weighed freeze-dried samples were soaked in deionized distilled water, and then washed through 63 µm sieves. The > 63 µm fraction residue was collected, dried, and then weighed. The weight of > 63 µm fraction divided by the weight of the original sample gives the coarse-fraction in wt%. In the analyses of calcium carbonate (CaCO 3 ) and total organic carbon (TOC) concentrations, dried bulk sediment were first ground to a powder, and then two weighed samples (~ 0.1 gram) were measured using a LECO CS-244 carbon/sulfur analyzer. One of the two weighed samples was directly measured for total carbon (TC) content of the sediment. The procedure involves heating the sample at 850 and measuring the combustion products by infrared energy detector. The other sample was digested with 2.4 N HCl to remove carbonates. The carbonate-free residue was washed thoroughly with deionized distilled water, dried, and then the residual carbon was measured. This value represents the 2

3 TOC content of the sediment. The difference between the two carbon measurements (TC and TOC) gives the total inorganic carbon (TIC) content. CaCO 3 concentration is calculated by wt%caco 3 = wt%tic/12 100, assuming all of the inorganic carbon was present as calcite or aragonite. Precision of the analyses is better than 0.04% and 0.10% for working standard (bulk sediments from the SCS) and sediment samples, respectively (Table 1). The method used here is different from that on board the JOIDES Resolution, but yields comparable results (Fig. 1, Fig. 2, and Fig. 4). RESULTS Coarse fractions of most sediment samples in Site 1143 are less than 5 wt%, with only two values above this range (Fig 1). No obvious trend is observed within the data set we generated. CaCO 3 concentrations of Site 1143 vary between and 6.03 wt%. The sediment contents of CaCO 3 wt% are higher below 300 mcd (meters composite depth) than above, but gradually decrease upwards from about 60 wt% at 310 mcd to the minimum at 18 mcd. Generally the variability pattern of CaCO 3 wt% is different from that of coarse fraction. Most of the TOC concentrations we measured are less than 0.5 wt%, with only three values considerably greater than this range. The increasing trend from 200 mcd to coretop sediments mirrors that of CaCO 3, suggesting the CaCO 3 concentrations reflect varying amounts of noncarbonate hemipelagic sediment input, e.g., clastics, admixed volcanic ash (Wang, Prell, Blum, et al., 2000). Coarse fractions in Site 1146 range between wt% and 1.02 wt% (Fig. 2). In general, the values are low for sections below 420 mcd and upper 200 m. It can t be ruled out, however, that this is an artifact resulted from the different sampling resolution along the core for this study. The concentrations of CaCO 3 wt% in Site 1146 we generated vary between 28 wt% and 18 wt% below 570 mcd, and then increase gradually to reach a plateau at about 415 mcd. The high contents of CaCO 3 wt% remain upwards till the depth at about 255 mcd, and then decline to a minimum at the top, with an exception of the obvious peak in between. Apart from some differences for details, the downcore record of coarse fractions mimics that of CaCO 3 wt% in Site 1146, implying the carbonate shells of microfossils are the major contributors for the sand fraction in core sediments. TOC values at Site 1146 are consistently lower than 0.3 wt% from the depth of ~200 mcd downwards to the bottom of the core. A progressive increase 3

4 of TOC occurs between 200 and 60 mcd, followed by the rapid increase to the maximum of 1.21 wt% at the top. A tentative comparison between the time series of these two records was made according to the preliminary age models developed based on the biostratigraphy and paleomagnetism measured onboard (Fig. 3; Wang, Prell, Blum, et al., 2000). Because there are only 7 data points older than 10 Ma for Site 1146 obtained from this study and the depositional history of Site 1143 is only 9 Myr, we compare these three parameters of the two sites for the last 10 Myr as shown on Figure 3. In general, the coarse fractions and CaCO 3 wt% of Site 1146 are higher than that of Site 1143 for sediments older than Quaternary. Besides, it is noticed that the CaCO 3 wt% of Site 1143 starts to decrease gradually since 7 Ma, yet Site 1146 remains constant till 4 Ma and then decreases rapidly until ~1 Ma (Fig. 3). Of special interest is that although the CaCO 3 contents are similar in late Miocene sections between these two sites, the accumulation rates of CaCO 3 at Site 1143 are about twice higher than that at Site 1146 (Fig. 4). The high accumulation rates of CaCO 3 in Site 1143 during late Miocene should be benefited from the contemporary increases in sedimentation rates, which was ascribed to the redeposition of adjacent sediments (Wang, Prell, Blum, et al., 2000). Whether this is the signature of the biogenic bloom recorded in the Equatorial Pacific regime (e.g., Berger et al., 1993; Farrell et al., 1995) waits for further confirmation. Furthermore, contrast to the monotonously decreasing in the accumulation rates of CaCO 3 for Site 1143 since 7 Ma (Fig. 4), accumulation rates of CaCO 3 from Site 1146 remain within a relatively narrow range throughout the record. Concentrations of TOC from Site 1146 are lower than Site 1143 (Fig. 3), except for the interval post to ~2 Ma. Nevertheless, the variation patterns are similar, with consistently low values prior to ~4 Ma and then increasing to the present. ACKNOWLEDGMENTS We are gratefully acknowledge the Ocean Drilling Program for providing the samples used in this study. Tai-Chun Lin is appreciated for her assistance in processing samples. This study was funded by the Taiwan National Science Council (grants NSC M ODP and NSC M ODP to H.-L. Lin). 4

5 REFERENCES Berger, W.H., Leckie, R.M., Janecek, T.R., Stax, R., and Takayama, T., Neogene carbonate sedimentation on Ontong Java Plateau: highlights and open questions. In Berger, W.H., Kroenke, L.W., Mayer, L.A., et al., Proc. ODP, Sci. Results, 130: College Station, TX (Ocean Drilling Program), Farrell, J.W., Raffi, I., Janecek, T.R., Murray, D.W., Levitan, M., Dadey, K.A., Emeis, K.-C., Lyle, M., Flores, J.-A., and Hovan, S., Late Neogene sedimentation patterns in the eastern equatorial Pacific. In Pisias, N.G., Mayer, L.A., Janecek, T.R., Palmer-Julson, A., and van Andel, T.H. (Eds.), Proc. ODP, Sci. Results, 138: College Station, TX (Ocean Drilling Program), Meyers, P.A., Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes. Org. Geochem., 27: Wang, P., Prell, W.L., Blum, P., et al., 2000, Proc. ODP, Init. Repts., 184, [CD-ROM]. Available from: Ocean Drilling Program, Texas A&M University, College Station TX , USA. 5

6 Figure Legends 1. Coarse fractions, CaCO 3 concentrations, and TOC concentrations in sediment samples from Site Sediment depths are given in meters composite depth (mcd). Solid symbols are data we generated for this study, and open symbols are onboard data from the Initial Report (Wang, Prell, Blum, et al., 2000). 2. Coarse fractions, CaCO 3 concentrations, and TOC concentrations in sediment samples from Site Sediment depths are given in meters composite depth (mcd). Solid symbols are data we generated for this study, and open symbols are onboard data from the Initial Report (Wang, Prell, Blum, et al., 2000). 3. Comparisons of coarse fractions, CaCO 3 concentrations, and TOC concentrations between sediment samples from Sites 1143 and Solid symbols are samples from Site 1143, and open symbols represent data for Site Age for individual sample was estimated based on control points from the Initial Report (Wang, Prell, Blum, et al., 2000). 4. Mass accumulation rates of CaCO 3 from Sites 1143 and Solid symbols are data of mass accumulation rates of CaCO 3 we generated for this study, and open symbols are onboard data from the Initial Report (Wang, Prell, Blum, et al., 2000). 6

7 Table Legend 1. Weight percentages of coarse fractions, total carbons, TOC concentrations, total inorganic carbons, and CaCO 3 concentrations in sediment samples for Sites 1143 and

8 10 Site 1143 Coarse fraction (>63 µm, wt%) CaCO 3 (wt%) TOC (wt%) New data Onboard data Depth (mcd) 8

9 25 Site 1146 Coarse fraction (>63 µm, wt%) CaCO 3 (wt%) TOC (wt%) New data Onboard data Depth (mcd) 9

10 25 Coarse fraction (>63 µm, wt%) CaCO 3 (wt%) TOC (wt%) Site 1143 Site Age (Ma) 10

11 6 5 Site 1146 New data Onboard data CaCO 3 MAR (g/cm 2 -ky) Age (Ma) 6 Site 1143 CaCO 3 MAR (g/cm 2 -ky) Age (Ma) New data Onboard data 11

12 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) A- 1H1, * (4) (3) H3, * (3) (3) H3, * (3) (3) H3, * (5) (3) H3, * H3, * H3, * (5) (3) H3, * H3, * (3) (2) H3, H3, * (5) (2) H4, H6, H1, H3, * (3) (3) X1, X3, X1, X3, X1, X3, X3, * X1, X1, X3, X5, (2) X1, X1, * (5) (4) X1, X3, X5, X1, (2) (2) X3, X5, X1,

13 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) 30X3, X3, * (2) (2) X5, X1, X3, X5, X1, X3, (3) X5, (3) X1, X3, (3) X3, * X5, X1, X3, X5, X1, X3, (2) X5, X1, X3, X3, * (4) (3) X5, X1, X3, X5, X1, X3, X5, (3) X1, X3, (3) (3) X3, * (4) (3) X5, X1, (3) X3, (3) X5,

14 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) 42X1, X3, X3, * (2) (2) X5, X1, (2) X3, (4) (2) X3, * (2) (3) B- 11H1, H2, (3) H4, (3) (2) H2, H4, H1, H3, H5, H3, H1, (2) H1, H3, H5, (3) H5, H1, H3, (3) X2, X4, C- 10H4, (2) H4, (2) (2) H5, H6, H5, (3) (3) H4, (2) (3) H1, H3, H5,

15 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) 18H5, H4, X1, X3, X3, * (3) (2) X5, X3, * (4) (2) X3, * (3) (3) X5, * (3) (3) A- 1H2, * (4) (4) H3, * (3) (3) H3, * (5) (2) H3, * (2) (2) H3, * (4) (2) H3, * H3, * (4) (3) H3, * (3) (4) H3, * (2) (2) H3, * (4) (2) H3, * (4) (2) H3, * (4) (2) H3, * (3) (3) X2, X3, X4, X5, X3, X3, * (2) (2) X3, * (2) (2) X5, X6, (2) X1, (3) X3, * (3) (4) X1, X2, (2)

16 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) 32X3, X4, X5, X6, X1, X2, (2) (2) X3, (3) X3, * (4) (3) X4, (2) X5, X6, X1, X2, X3, (2) X3, X4, (2) X5, X6, X2, (3) X3, X3, * (4) (2) X4, X5, X3, (2) X4, X5, X3, X4, X5, (2) X6, X2, X3, X3, * (2) (2) X4, (3) X5, X4,

17 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) 41X2, X3, X4, X5, X6, X1, X2, X3, X3, * (3) (4) X4, X5, (2) X6, X3, * (2) (3) X3, * (3) (2) X3, * (2) (3) X3, * (2) (2) X3, (3) (2) X3, * (2) (2) X3, * (2) (2) B- 21H3, H4, H5, H2, H3, H4, H5, X1, (2) (2) X2, X3, X4, X2, X3, (3) (4) X4, X5, (2) X6,

18 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) C- 20X5, (2) (2) X6, (2) X4, X5, X6, X2, (2) (2) X3, X4, (3) (2) X5, (3) (2) X2, X3, X4, X5, X6, X1, X2, X3, X4, X5, X6, (2) X1, X2, X3, X4, (2) X5, X1, X2, X3, X4, X5, X6, X1, X2, (2) (2) X3, X4,

19 Coarse TIC Core, section, Depth fraction TC Stdev TOC Stdev (TC-TOC) CaCO 3 interval (cm) (mcd) (wt%) (wt%) (1σ) (wt%) (1σ) (wt%) (wt%) 29X5, X6, X1, X2, X3, X3, X3, X3, X4, (2) X5, X6, X3, X4, X5, X3, X4, X5, X3, X4, X5, X2, X3, X4, (2) X5, X1, X2, X3, X1, (3) X2, X3, X4, X1, X2, X3, Note: 1. Symbol * indicates samples of squeezed cakes. 2. Numbers in parenthesis represent the analytical times for individual sample. 19