**8. Organic matter (OM) and stable carbon isotope (δ13C)**

 The inner continental shelf extended from Rio Soto La Marina River and the Laguna Madre represented vital deposition area of sedimentary organic matter of continental origin. The applied geochemical analysis revealed significant shifts in the concentrations of organic matter (OM) and organic carbon (OC) throughout the 3-year period of observation, showing a progressive increase over time. However, the values remained within the known ranges of concentrations previously recorded in the Gulf of Mexico. No significant changes were detected in the spatial pattern of distribution of organic inputs during the three periods of observation. However, the estimated OC percentages did show significant variability over time; such variability was more evident in deep sites (>1000 m), where presumably, there is a substantial accumulation of OM caused by processes of deposition or sediment transport on the continental slope.

 The δ13C values during the three campaigns fluctuated between −20.16 and −21.66‰ with an average of −21.02 ± 0.34‰. There seems to be and impoverished gradient of δ 13C values from the northwestern corner close to the coast, which gradually increased outward to the oceanic region, following a southeast pathway. The δ13C results highlighted the predominance of autochthonous organic matter (marine) as the primary source of sedimentary OC over that of terrigenous origin, particularly at remote sites from the coast. However, as expected, near the coast, where there are important inputs of terrestrial organic matter derived primarily from C4 plants, the OC isotopic signature is masked by the mixture with the

autochthonous organic matter. In the present investigation, the isotopic fingerprint belonging to the Deepwater Horizon oil spill (−27.23 ± 0.03‰ for weathered petroleum and −27.34 ± 0.34‰ for crude oil) was not detected.
