Research Papers Volumetric budget calculation of sediment and carbon storage and export for a late Holocene mid-shelf mudbelt system (NW Iberia) Ferdinand K.J. Oberle n , Till J.J. Hanebuth, Benjamin Baasch, Tilmann Schwenk MARUM – Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany article info Article history: Received 25 February 2013 Received in revised form 19 December 2013 Accepted 30 December 2013 Available online 8 January 2014 Keywords: Continental shelves Mud depocenter Holocene sediment budget Shelf sediment transport Carbon cycle Iberia abstract 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 km 3 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 CaCO 3 amount to 40.31 to 43.46 t and 174.73 to 186.68Mt, respectively. High-resolution isopach analysis shows high regional morphodynamic variability of the main sediment transit routes, an aspect easily over- looked 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. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction In the study of source-to-sink systems, continental shelves play a major role because they host permanent and temporary depocenters. These depocenters occupy a pivotal position within the entire source- to-sink system, which consists of multiple, genetically linked seg- ments (coastal lowlands, shelf, slope, and deep ocean basin) (Moore, 1969). Consequently, erosion or deposition in one part of the system leads to sediment-storage modifications within the adjacent seg- ment. Because continental shelf systems vary greatly in tectonic setting, local physiography, hydrodynamic conditions, and sediment supply (type, volume, and place of input), among other factors, a general understanding must be based on numerous system-specific studies. Many of the involved processes—ranging from sediment delivery to gravity flows—that give a shelf its morphological char- acter have been documented for narrow active continental margins and wide passive continental margins with high sediment input from rivers (Nittrouer et al., 2007; Gerber et al., 2010). The current study presents the sedimentary budget of a typical narrow passive con- tinental shelf having low sediment input from rivers and a globally representative mud depocenter. On modern storm- and wave-dominated siliciclastic continental shelves, mud depocenters (“mudbelts”) are common depositional elements that represent the most proximal main sinks for fluviogenic material supplied to the ocean. The individual development of a mud depocenter depends on a focused sediment supply and the specific hydrographic conditions interacting with the local morphology (McCave, 1972; McKee et al., 2004; Nittrouer et al., 2007). As is true globally for most mid-shelf mudbelts, the NW Iberian mudbelt is due to sedimentary buildup resulting from post-glacial shelf drowning caused by Holocene transgression and the subsequent sea-level highstand (stabilization) that results in today 0 s relatively weak hydrodynamic conditions (e.g., Grossman et al., 2006). More speci- fically Nittrouer and Sternberg (1981) have shown that sediment deposition on a narrow high energy shelf is most effective when peak river flow and maximum oceanic dispersal are out of phase. Sediment transport on the shelf is controlled by a number of autogenic and allogenic factors such as sediment concentration, morphology (e.g., width and gradient), wave regime, wind climate (e.g., direction, frequency and intensity), and tidal and alongshore currents (Nittrouer and Wright, 1994; Sánchez-Arcilla and Simp- son, 2002). These factors vary laterally, so mud deposition and remobilization within a shelf environment also vary (e.g. Spinelli and Field, 2003). Budgetary calculations for sedimentary deposi- tion on shelves that employ high-density data grids and isopach analysis (e.g., Eittreim et al., 2002; Brommer et al., 2009; Miller Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.12.012 n Corresponding author. Tel.: þ49 421 218 65952; fax: þ49 421 218 65515. E-mail address: foberle@marum.de (F.K.J. Oberle). Continental Shelf Research 76 (2014) 12–24