Mass accumulation rate of organic carbon in relation to the global climatic changes in the western equatorial Pacific during the late Quaternary
Nobuhisa Eguchi1, Hodaka Kawahata2 and Asahiko Taira1
1 .Ocean Research Institute, University of Tokyo
2. Marine Geology Department, Geological Survey of Japan
Key words: Palaeoproductivity, MARoc, Global climatic changes
ABSTRACT
Four sedimentary cores were collected for the study of the palaeoproductivity and its linkage to the climatic changes in the western equatorial Pacific.
The average Corg/N atomic ratio of the sediments is 6.8 which indicates that the organic matter in the sediments are mainly of marine origin. We use mass accumulation rate (MAR) of organic carbon as a proxy of the palaeoproductivity in this study.
We show MARoc and oxygen isotope profiles of the four cores. The stable oxygen isotope is regard as the paleoclimalic indicator. In the western equatorial Pacific modern primary productivity is higher at the west Caroline basin and lower at the north Fiji Basin. During the last 300 kyr, the similar trend has existed in the western equatorial Pacific. Organic carbon data from the West Caroline basin and the Eauripik rise show climatic controlled fluctuations although the Ontong-Java plateau and the North Fiji Basin do not show such trend. It may suggest that the oceanography of the western equatorial Pacific has not remarkably changed during the last 300 kyr.
1. INTRODUCTION
The production of organic carbon and carbonate in the oceans is an important subject in the global carbon cycle. Surface ocean productivity can act as a "biological pump" which vertically redistributes carbon in the oceans and draws down surface water CO21).
Mass accumulation rate (MAR) of organic carbon is one of the good indicator for the past ocean primary productivity.
Here we present the MARoc of four sedimentary cores taken from the western equatorial Pacific and try to find their response to climatic fluctuations.
2. SEDIMENTS AND METHODS
2-1. Sediments
The samples used for this study were taken in the western equatorial Pacific during the cruises of NH9O-1 and NH92-I by RN HAKUREI under the auspices of the "Northwest Pacific Carbon Cycle Study (NOPACCS)" which is undertaken by the "New Energy and Industrial Technology Development Organization (NEDO)"(Fig. 1). C4402 was collected from a depth of 4,402 m at 135°01.3’E 2°59.8'N in the West Caroline Basin. The sediments were siliceous carbonate oozes. NGC34 was collected from a 2,188 m water depth at 141"29.44E, 3°48.09'N on the Eauripik Rise which was located at the eastern part of the West Caroline Basin. The 334cm long core consisted mainly of dull yellow orange to light olive gray calcareous ooze intercalated with some greenish gray silt layers2). NGC36 was retrieved from a depth of 3,187 m at 160°34.4'E 1°13.2'N on the Ontong Java Plateau which was located at east of the East Caroline Basin. The 564cm long core is mainly composed of oxidized dull yellow orange calcareous ooze and light gray bioturbated calcareous ooze2). NGC38 was recovered from a depth of 3,002 m at 175°09.87'E, 14°59.41'S in the North Fiji Basin which located northeast of the Fiji Islands. The 390cm long core consisted mainly of brown-yellowish calcareous clay intercalated with two volcanic ash layers2).
2-2. Analytical methods
Separate samples were taken for foraminiferal isotopic and for chemical analyses. Sediment aliquots for chemistry were frozen on board and kept at -20'C before analysis in a land-based laboratory.
