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Integrated Ocean Drilling Program (IODP) Expedition 334 drilled slope sediments on the Costa Rica subduction zone to understand the seismogenic processes in an erosional subduction zone. Evidence for tectonic erosion in this area back to ~16 Ma has been reported (Vannucchi et al., 2001). One of the primary goals of the expedition was to reveal the time evolution of tectonic erosion in this area, so dating of the sediment is of great importance. Although preliminary biostratigraphy and magnetostratigraphy suggest very high sedimentation rates (51.6–23.6 cm/ky for Site U1378 and 103.5–16.0 cm/ky for Site U1379; see the “Expedition 334 summary”; chapter [Expedition 334 Scientists, 2012a]), shipboard paleomagnetic measurements could not fully isolate primary remanence because of severe drilling-related overprint, and complete magnetostratigraphy awaits detailed demagnetization experiments.

Rock magnetic characterization of sediments is essential to the interpretation of paleomagnetic recordings. For example, it is widely known that diagenetic reduction of magnetic iron oxide degrades the magnetic recording in sedimentary environments where bacterial reduction of sulfate occurs (e.g., Karlin and Levi, 1983; Berner, 1981; Tarduno, 1994; Rowan et al., 2009; Dewangan et al., 2013). Importantly, diagenetic reduction of iron oxide does not always follow the present-day chemical profile of pore water; past migration of the chemical reaction front could produce complex zonation in the diagenetic product (Rowan et al., 2009; Dewangan et al., 2013) that in turn results in complex smoothing of the paleomagnetic record (Rowan et al., 2009). Sediments from Sites U1378 and U1379 contain pyrite, indicating sulfate reduction has also taken place in those sediments. To help constrain the impact of such processes on paleomagnetic recordings, I conducted rock magnetic measurements on sediment samples from Sites U1378 and U1379.