Naomi Harada

Origin and decomposition of sinking particulate organic matter in the deep water column inferred from the vertical distributions of its δ15N, δ13 and δ14

Sinking particles were analyzed for their nitrogen isotopic ratio δ15N) of total particulate nitrogen (PN), stable carbon isotopic ratio (δ13C) and radioactive isotopic ratio (δ14C) of total particulate organic carbon (POC), at three different latitudinal (temperate, subpolar and equatorial) and geomorphological (trench, proximal abyssal plain and distal abyssal plain) sites in the western North Pacific Ocean using year-long time series sediment trap systems, to clarify the common vertical trends of the isotopic signals in deep water columns. Although the δ15N and δ13C values of sinking particulate organic matter (POM) were partly affected by the resuspension of sedimentary POM from the sea floor, especially in the trench, the changes in δ15N and δ13C values owing to the resuspension could be corrected by calculation of the isotopic mass balance from δ14C of sinking POC. After this correction, common downward decreasing trends in δ15N and δ13C values were obtained in the deep water columns, irrespective of the latitudes and depths. These coincidental isotopic signals between δ15N and δ13C values provide new constraints for the decomposition process of sinking POM, such as the preferential degradation of 15N- and 13C-rich compounds and the successive re-formation of the sinking particles by higher trophic level organisms in the deep water column.

Glacial‐interglacial migration of an upwelling field in the Western Equatorial Pacific recorded by sediment 15N/14N

Nitrogen isotope ratios (δ15N) of surface sediment in the equatorial ocean reflect the mode of upper water circulation pattern. We applied this knowledge to reconstruct glacial-interglacial variations of an upwelling field in western equatorial Pacific using δ15N from sediment cores. Distinct minima in δ15N values were found corresponding to glacial periods on the northern and eastern edges of the upwelling field (Mindanao Dome), indicating glacial enlargement of upwelling due to enhanced winter monsoon activity. However, at a site on southern side of the upwelling region, minima in δ15N values were obtained during interglacial periods. These opposing variations in δ15N values could be interpreted as a northward shift of the upwelling zone, during glacial periods. The glacial-interglacial migration of the circulation pattern in this region may reflect a modal change in the global thermohaline circulation (conveyor belt) during glaciation.

Source of hydrocarbons in marine sediments in Lützow-Holm Bay, Antarctica

Abstract We have made an attempt to reconstruct the paleoenvironment of the coastal region around Syowa Station in Lutzow-Holm Bay, Antarctica, using organic compounds in marine sediments. A 129 cm long core sample of the oceanic sediment was collected at 69°14′48″S, 39°11′33″E in Lutzow-Holm Bay. 14C age showed a smooth pattern of change from about 3000 yBP at the surface to 14,000 yBP at the bottom, except for a change of age in the range of 80–120 cm depth. n-C21 was dominant in the low hydrocarbon content sections at the depths of 0–5, 60–65 and 120–125 cm of the core, whereas long-chain alkanes (C22, C23, C24) were dominant in the high hydrocarbon content sections at the depths of 45–50 and 90–95 cm. These characteristic distribution patterns of n-alkanes suggested that their major source probably is mixed altered and/or recycled material and recent organic material possibly derived from marine organisms. The differences in the δ13C values of individual alkanes from the high and low hydrocarbon content sections of the core were not appreciable, suggesting a common source for these compounds. δ13C values for organic matter in different sections of the core range from −26.0 to −28.5‰, indicating that its major source is marine organisms.

Chronology of marine sediments by the racemization reaction of aspartic acid in planktonic foraminifera.

Abstract Racemization reaction rate constants of aspartic acid (kASP) were obtained and compared in bulk foraminifera and in a dissolution resistant foraminiferal species Pulleniatina obliquiloculata obtained from marine sediments through ca. 20,000 yrBP in the western equatorial Pacific. The average kAsp values were 0.63 × 10−5 yr−1 for bulk foraminifera and 0.72 × 10−5 yr−1 for P. obliquiloculata. This disparity in kAsp values could be attributed to the presence of dissolution vulnerable foraminifera in bulk foraminifera. It is believed that the leaching of racemized free amino acid (having high D L values) when the calcareous test was subjected to dissolution through geological time could have led to a lower value of kAsp in bulk foraminifera. This study thus demonstrates that it is important to analyze a single species of planktonic foraminifera resistant to dissolution in order to avoid spurious changes in kAsp values. If this condition were satisfied, aspartic acid racemization is shown to be extremely useful, not only for determining the geological age of marine sediment, but also to assess the quality of the sedimentary core stratigraphy.

Amino acid chronology in the fossil planktonic foraminifera, Pulleniatina obliquiloculata from Pacific Ocean

This study was undertaken to evaluate the accuracy of amino acid chronology using a first-order kinetic model and was carried out using a fossil planktonic foraminifera, Pulleniatina obliquiloculata (P. obliquiloculata), in the sediment from the western equatorial Pacific Ocean. The extent of racemization reaction of aspartic acid and glutamic acid and the extent of epimerization reaction of isoleucine were measured and compared with reference ages obtained from 14C dating and the distribution patterns of δ18O. The relationships between the extent of racemization/epimerization reaction of these amino acids and reference ages were linear along with a high correlation coefficient (r = 0.99–0.95), suggesting that amino acid chronology utilizing a first-order kinetic model could be appropriate for determining geological ages ranging from the present to 106 yBP within the average errors of 4.5, 12.3, and 15.8% for Asp, GIu, and Ile, respectively.