Volumetric budget calculation of sediment and carbon storage and export for a late Holocene mid-shelf mudbelt system (NW Iberia)

• First high-resolution isopach study of the NW Iberian mudbelt.
• Rare late Holocene sediment-budget analysis of a passive, narrow continental shelf.
• 35% of all sediment entering the shelf through rivers stay on the shelf.
• Carbon content of entire mudbelt is (volumetrically) calculated.
• Novel method of using sediment grain size as a proxy for isopach depth.

Confined fine-grained depoenters (mudbelts) on continental shelves play an important role as common and major fluviogenic submarine depocenters along the source-to-sink pathway and in global sedimentary and carbon cycles. This study provides a complete high-resolution isopach-based budget analysis using closely-spaced, high-resolution seismic-reflection data of an exemplary mid-shelf mudbelt system located on the open and narrow continental shelf of NW Iberia.The budget analysis reveals that 3.957–4.227 km3 of sediments [i.e., 4073 to 4351 Mt (dry)] are stored in this depocenter. In conjunction with river-discharge estimates, we calculate that, over the past 5300 yr, approximately 34% to 36% of total fluvial sediments supplied to the ocean remain in the shelfal mud depocenter and the balance bypasses the shelf. Total accumulation values for TOC and CaCO3 amount to 40.31 to 43.46 t and 174.73 to 186.68 Mt, respectively. High-resolution isopach analysis shows high regional morphodynamic variability of the main sediment transit routes, an aspect easily overlooked by core-based or low-resolution profiling studies. The budget analysis reveals persistent low accumulation over the past 5300 yr and thus clarifies that a uniformitarian view of applying modern accumulation rates to the late Holocene can significantly underestimate effective sediment off-shelf transport.