Organic carbon production, mineralisation and preservation on the Peruvian margin

Abstract

Abstract. Carbon cycling in Peruvian margin sediments (11 and 12° S) was examined at 16 stations, from 74 m water depth on the middle shelf down to 1024 m, using a combination of in situ flux measurements, sedimentary geochemistry and modelling. Bottom water oxygen was below detection limit down to ca. 400 m and increased to 53 μM at the deepest station. Sediment accumulation rates decreased sharply seaward of the middle shelf and subsequently increased at the deep stations. The organic carbon burial efficiency (CBE) was unusually low on the middle shelf (<20%) when compared to an existing global database, for reasons which may be linked to episodic ventilation of the bottom waters by oceanographic anomalies. Deposition of reworked, degraded material originating from sites higher up on the slope is proposed to explain unusually high sedimentation rates and CBE (>60%) at the deep oxygenated sites. In line with other studies, CBE was elevated under oxygen-deficient waters in the mid-water oxygen minimum zone. Organic carbon rain rates calculated from the benthic fluxes alluded to efficient mineralisation of organic matter in the water column compared to other oxygen-deficient environments. The observations at the Peruvian margin suggest that a lack of oxygen does not greatly affect the degradation of organic matter in the water column but promotes the preservation of organic matter in sediments.

Strong and Dynamic Benthic-Pelagic Coupling and Feedbacks in a Coastal Upwelling System (Peruvian Shelf)

“…These authors further attributed the H 2 S plume to benthic H 2 S release, and estimated a benthic H 2 S source of around 2 mmol m −2 d −1 on the middle shelf at 100 m water depth. This is in agreement with the present model prediction (0.4 mmol m −2 d −1 ) when one considers that organic matter mineralization at 100 m is a factor of 3-4 higher than at 150 m (Dale et al, 2015). The model results suggest that large-scale H 2 S emission events may occur more regularly than currently presumed and may have gone undetected in 2004, 2005, and 2006 (Figure 5F).…”

“…From the same study, the sediment accumulation rate and bioturbation coefficient were determined by radiotracer measurements to be 0.15 cm year −1 and 2 cm 2 year −1 , respectively. The resulting simulated surface POC content of 5.7 wt.% ( Figure 2B, dotted red curve) is close to the measured value on the outer shelf of 7.5 ± 3.2 wt.% (Dale et al, 2015). The carbon burial efficiency is 50%, which is well within the range of 24-74% determined for these stations based on measured POC accumulation rates and in situ flux measurements of dissolved inorganic carbon.…”

“…FIGURE 1 | (A) Peru margin and sampling locations. The location of the IMARPE time-series station (150 m water depth, red symbol) and the sampling stations at 11 • S and 12 • S (black symbols) where organic carbon rain rates were determined by Dale et al (2015) are indicated (see Figure 3C). Monthly satellite data (1/6 degree resolution) were used to determine mean PP and EP fluxes for the ocean area shown (77 to 79 • W and 10.5 to 13 • S).…”

“…The model simply calculates the POC content for the imposed RPOC profile, POC rain rate and transport processes. The rain rate was taken as 17 mmol m −2 d −1 (204 mg m −2 d −1 ), which is the average rain rate determined for the outer shelf stations (Dale et al, 2015). From the same study, the sediment accumulation rate and bioturbation coefficient were determined by radiotracer measurements to be 0.15 cm year −1 and 2 cm 2 year −1 , respectively.…”

“…Only the least reactive fraction survives below ca. 10 cm, and is thus mainly responsible for the 50% carbon burial efficiency in the OMZ (Dale et al, 2015).…”