C:N:P Molar Ratios, Sources and 14C Dating of Surficial Sediments from the NW Slope of Cuba.

Guadalupe De La Lanza Espino,Luis A Soto

doi:10.1371/journal.pone.0125562

Abstract

The surficial sediments recovered from 12 sites located near the channel axis of the Florida Straits and the lower slope off NW Cuba were analyzed for total organic carbon (TOC), nitrogen (TN), phosphorus (TP), elemental C:N:P ratios, C and N isotopic values, and 14C dating. The depth profiles of TOC, TN, and TP (0-18 cm) displayed a downcore trend and a significant variation. The TOC values were low (0.15 to 0.62%; 66 to 516 µmol g-1). Sites near the island’s lower slope had lower TOC average concentrations (158-333 µmol g-1) than those closer to the channel axis (averaging 341-516 µmol g-1; p <0.05). The TN concentrations near the lower slope attained 0.11% (80 µmol g-1), whereas, towards the channel axis, they decreased to 0.07% (55 µmol g-1; p<0.05). The C:N ratios ranged from 1.9 to 10.2. The mean molar C:N ratio (5.4) indicated a marine hemipelagic deposition. The TP was lower at sites near the lower slope (38.4 to 50.0 µmol g-1; 0.12% to 0.16%) than those near the channel axis (50.0 to 66 µmol g-1; 0.15 to 0.21%). C:P fluctuated from 7.7 to 14.1 in the surficial sediment layer. The bulk organic δ13Corg and δ15N values confirmed pelagic organic sources, and the 14C dating revealed that the sediments were deposited during the Holocene (1000-5000 yr BP). We suggest that the hydrodynamic conditions in the Straits influence vertical and advective fluxes of particulate organic material trapped in the mixed-layer, which reduces the particulate matter flux to the seabed.

Highlights

The carbon, nitrogen and phosphorus contents and their molar ratios in sea-water and marine sediments are regularly used to determine the origin and transformation of organic matter (OM)
The analyzed subcores can be classified as organic-poor slope carbonate sediments, in which terrestrial input is negligible and from a region characterized by its low surface productivity
The hydrodynamic conditions prevailing in the oligotrophic waters of the Straits of Florida must exert a significant influence upon the vertical and advective fluxes of particles of organic matter (POC) trapped in the mixed-layer, causing a low OM input to the bottom

Summary

Introduction

The carbon, nitrogen and phosphorus contents and their molar ratios in sea-water and marine sediments are regularly used to determine the origin and transformation of organic matter (OM). These ratios can be influenced by a series of environmental factors highlighted by climate, terrigenous input, bathymetry, and bottom circulation [1,2,3,4,5,6,7,8,9]. There is evidence that organic C and N contents display decreasing trends in sediments along depth gradients (200 to 3000 m), such as off the Mississippi River Delta in the Gulf of Mexico [3] In this region, the sedimentary OM derived from the river runoff is more depleted in N and P than that from a PLOS ONE | DOI:10.1371/journal.pone.0125562. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials

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