Changes in the sedimentary organic carbon pool of a fertilized tropical estuary, Guanabara Bay, Brazil: an elemental, isotopic and molecular marker approach

• Published in: Marine chemistry Vol. 79; no. 3; pp. 207 - 227
• Main Authors: Carreira, Renato S., Wagener, Angela L.R., Readman, James W., Fileman, Tim W., Macko, Stephen A., Veiga, Álvaro
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 01-10-2002; Elsevier Science
• Subjects: Brackish; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Geochemistry; Guanabara Bay; Isotope geochemistry; Isotope geochemistry. Geochronology; Isotopes; Mineralogy; Organic carbon; Pollution, environment geology; Sediments; Silicates; Steroids; sterols; Tropical estuary; Water geochemistry; Brazil; South Atlantic; Rio de Janeiro Brazil; Guanabara Bay; Southwest Atlantic; ASW, Brazil; Brazil, Guanabara Bay; ASW, Brazil, Rio de Janeiro, Guanabara Bay; Organic carbon; Guanabara Bay; Tropical estuary; Sediments; Isotopes; Steroids; South America; stream transport; stable isotopes; estuarine environment; mass balance; C-13/C-12; N-15/N-14; Atlantic Ocean; concentration; sterols; pollution; sedimentation rates; factor analysis; drill cores; hydrochemistry; water quality; organic materials; estuaries; steroids; organic carbon
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/S0304-4203(02)00065-8

Sediments from Guanabara Bay, sampled as short-cores (50–60 cm) from eight stations, were analysed for the elemental (C and N), isotopic (
δ
13C and
δ
15N) and molecular composition (steroids) of the organic matter. The objective of this study was to examine whether there are changes in the sources of sedimentary organic matter pool due to increasing N, P and particulate loads to the bay over the last 100 years. On average, we found a 10-fold increase in the flux of organic matter to the sediments with a maximum of 41.7 mol C m
−2 year
−1. C/N and
δ
13C time–space distribution and mass balance calculations indicate that the organic material results from a mixture of marine, terrestrial and estuarine sources, with a growing predominance of autochthonous inputs in recent years. Dinosterol was predominant in most samples (>50% of all measured sterols). Concentrations of dinosterol increased significantly in the last two decades and reached values as high as 150 μg g
−1 dry sediment. Net increases in concentration were estimated using the first-order decay rate constants derived from experimental data. There are strong indications that an increase in carbon storage occurred in response to growing eutrophic conditions. In spite of the high respiration rates occurring in the water column, fast sedimentation rates result in the transfer of a significant carbon fraction to the anoxic sediments. Further decomposition proceeds only in the first 10 cm of sediment as indicated by the decay rates.