Geo-Marine Letters (1996) 16:36-40 © Springer-Verlag 1996 M. A. Allison • C. A. Nittrouer • L. E. C. Faria, Jr.. O. M. Silveira • A. C. Mendes Sources and sinks of sediment to the Amazon margin: the Amapa coast Revision received: 16 March 1995 Abstraet Modern and Holocene muddy strata were stud- ied along the shoreline adjacent to the Amazon river mouth using sedimentological, radiochemical, physical, and seismic methods. The present paper is a synthesis of the results, collected during the AmasSeds project, that is used to outline a regional shoreline sediment budget. Erosion of relict Amazon muds in southern Amapa sup- plies 10 6 tons yr -1 to the Amazon advective mud stream. Local rivers are sediment-poor (total suspended discharge 1 X 10 6 tons yr-1), but form depositional sandflats on the shoreface downdrift of the river mouths. Mudflat accu- mulation in northern Amapa sequesters 106-107 tons yr -~ by tidal-flat aggradation, alongshore mudcape aecretion, and sediment trapping by mangroves. The processes tem- porarily store 1.5 x 108 tons of Amazon mud in Janu- ary-June. Introduction Prior to AmasSeds (Nittrouer et al. 1991), the budgetary calculation of sediment accumulation in the Amazon sub- aqueous delta (Kuehl et al. 1986; Nittrouer et al. 1986) indicated that the adjacent shoreline of the Brazilian state of Amapa (Fig. 1) might be the sink for as rauch as one third of the total Amazon sediment load of ~ 1.2 x 109 tons yr -1 (Meade et al. 1985). This provided the moti- M. A. Allison 1 ([~~) • C. A. Nittrouer Marine SciencesResearch Center, State University of New York, Stony Brook, New York 11794-5000, USA L. E. C. Faria, Jr. • O. M. Silveira • A. C. Mendes PROMAR, Departamento de Geologia, Universidade Federal do Para, Av. Perimetral s/n 66050 Belem-Para,Brazil Present address: 1Department of Oceanography, Texas A&M University, 5007 Avenue U, Galveston, Texas 77551, USA vation to undertake a field program in Amapa synchro- nous with the AmasSeds shelf study. Results of the Amapa study have shown that, although the shoreline presently is sequestering only a fraction of the original estimate (Allison et al. 1995a), the mudflats are an important area of temporary Amazon sediment storage. In addition, shore- line erosion and sediment supply from local rivers are sources of sediment to coastal waters. The objective of this paper is to outline the mechanisms and volumes of sediment interchange between the Amapa coast and adjacent Amazon shelf identified during the joint US-Brazilian field program, in order to create a re- gional shoreline sediment budget. This synthesis also will integrate the improved understanding of shelf processes resulting from other AmasSeds studies. The detailed sedi- mentological and stratigraphic results of the Amapa study utilized in the present paper are presented in Allison et al. (1994; 1995a,b), Mendes and Faria (1993), Silveira et al. (1993), and Faria et al. (1993). Forcing mechanisms in the Amapa coastal zone The Amapa coast is an energetic setting with sediment dynamics driven by the interaction of several factors. The semidiurnal tidal range is large, decreasing from 5-8 m in the Cabo Norte-Ilha de Maraca region to 2-3 m at Cabo Cassipore (locations on Fig. 1), and produces shorenormal tidal currents (up to 35 cm s -t on the northern Amapa mudflats) (Allison et al. 1994). Beardsley et al. (1995) ascribe the large M2 tidal amplitude along the southern Amapa coast to near resonance that results from a con- cave shoreline trend and the shore-perpendicular Cabo Norte shoal. Tidal models (Beardsley et al. 1995) suggest amplification also may be produced by fluid muds on the shelf north of the shoal, which reduce the bottom drag coefficient for tidal-wave progression. Allison et al. (1995a) postulate that the south-to-north decrease in semidiurnal tidal amplitude (and thus, tidal currents) is responsible for a change from erosion along the southern Amapa