Radiocarbon studies of organic compound classes in plankton and sediment of the northeastern Pacific Ocean

Xu-Chen Wang,Ellen R.M Druffel,Sheila Griffin,Cindy Lee,Michaele Kashgarian

doi:10.1016/s0016-7037(98)00074-x

Abstract

Radiocarbon (Δ 14C) and stable carbon isotopes (δ 13C) were measured in total hydrolyzable amino acid (THAA), total carbohydrate (TCHO), total lipid, and acid-insoluble organic fractions that had been separated from phytoplankton, zooplankton, sediment floc, and sediment samples from an abyssal site in the northeastern Pacific Ocean. THAA, TCHO, and lipid fractions accounted for 91–99% of the total organic carbon (TOC) in phytoplankton and zooplankton, 57% of TOC in sediment floc, and 18–38% of TOC in sediment. Based on concentration profiles in sediment, first-order degradation rate constants below the bioturbation zone were calculated using a “multi-G” model considering both labile and refractory organic fractions. The calculated rate constants were in the order THAA ≈ TCHO > TOC ≈ TN > lipid, indicating the relative reactivities of these compound classes in the sediment during early diagenesis. Bioturbation affected the distributions of these compound classes in the top few centimeters of the sediment. The Δ 14C values of all organic fractions decreased in the order plankton in surface water to sediment floc to sediments at 4100 m depth. Distinct differences in Δ 14C exist among THAA, TCHO, and lipid fractions in sediment floc and sediments. The lipid fraction exhibited lower Δ 14C signatures than THAA and TCHO fractions. Differential decomposition of organic matter and sorption and/or biological incorporation of ‘old’ DOC into sediment appear to be the major processes that likely control the observed Δ 14C signatures and abundances. δ 13C values of the organic compound classes in sediment are similar to their values in plankton indicating that organic matter input to sediment in the northeastern Pacific is mainly from marine sources. Also, distinct δ 13C signatures were found in each of the four organic fractions.

Highlights

Carbon isotopic (⌬14C, ␦13C) analyses of organic matter provide a powerful tool for determining the sources and cycling of organic matter in the ocean (Williams and Linick, 1975; Druffel and Williams, 1992)
All organic matter recently produced in the surface ocean by photosynthesis and subsequently ingested by organisms is tagged with bomb 14C and is detected in subsurface organic carbon pools (Broecker et al, 1985; Druffel et al, 1992)
Williams et al (1992) used measurements of ⌬14C and ␦13C in the Santa Monica Basin, California, to show that terrestrial carbon accounted for 10% or less of the organic carbon in the sinking and suspended particulate organic matter (POM) and as high as 25% in the sediment

Summary

Introduction

Carbon isotopic (⌬14C, ␦13C) analyses of organic matter provide a powerful tool for determining the sources and cycling of organic matter in the ocean (Williams and Linick, 1975; Druffel and Williams, 1992). Bauer et al (1992) and Druffel et al (1992) found that DOC in the deep Pacific and Atlantic oceans has 14C ages of several thousand years, and that this value in the deep Pacific is about 2000 yr ‘older’ than that in the deep Atlantic. They suggested that this difference in DOC age between the two oceans is largely related to the deep-water circulation patterns and transit times between the oceans

Methods

Results

Conclusion