Episodic organic carbon fluxes from surface ocean to abyssal depths during long-term monitoring in NE Pacific

Abstract

Growing evidence suggests substantial quantities of particulate organic carbon (POC) produced in surface waters reach abyssal depths within days during episodic flux events. A 29-year record of in situ observations was used to examine episodic peaks in POC fluxes and sediment community oxygen consumption (SCOC) at Station M (NE Pacific, 4,000-m depth). From 1989 to 2017, 19% of POC flux at 3,400 m arrived during high-magnitude episodic events (≥mean + 2 σ), and 43% from 2011 to 2017. From 2011 to 2017, when high-resolution SCOC data were available, time lags between changes in satellite-estimated export flux (EF), POC flux, and SCOC on the sea floor varied between six flux events from 0 to 70 days, suggesting variable remineralization rates and/or particle sinking speeds. Half of POC flux pulse events correlated with prior increases in EF and/or subsequent SCOC increases. Peaks in EF overlying Station M frequently translated to changes in POC flux at abyssal depths. A power-law model (Martin curve) was used to estimate abyssal fluxes from EF and midwater temperature variation. While the background POC flux at 3,400-m depth was described well by the model, the episodic events were significantly underestimated by ∼80% and total flux by almost 50%. Quantifying episodic pulses of organic carbon into the deep sea is critical in modeling the depth and intensity of POC sequestration and understanding the global carbon cycle.

Keywords: abyssal community; carbon budget; carbon flux.

Fig. 1.

(A and B) Full Station M time series of sea surface temperature (A) and surface chlorophyll a concentration (green lines), net primary production (blue lines), and export flux (black lines) (B) estimated from satellites with a 100-km radius around Station M. (C–E) Trap-measured particulate organic carbon flux (C), detrital aggregate % cover on the sea floor (D), and sediment community oxygen consumption (E). Time-series mean, and mean + 2 σ threshold, are shown as solid and dashed lines, respectively.

Fig. 2.

Contribution of pulse events to annual carbon flux to 3,400-m depth, for years with >50% sampling coverage, based on sediment trap measures. Dark gray bars represent the proportion of annual POC flux that arrived during pulse periods, while light gray bars represent the proportion that arrived during nonpulse periods. Mean annual POC flux (mg C⋅m⁻²⋅d⁻¹) is given above each bar.

Fig. 3.

Time lags in days associated with pulse periods of EF (A), POC flux to 3,400 m (B), DA % cover (at 4,000 m) (C), and SCOC (at 4,000 m) (D). Red lines identify periods when values exceeded the 1989-to-2017 time-series mean + 2 σ (dotted lines), the threshold for pulse events used in the analysis. Graphics below the time-series plots show timing of peaks in cross-correlations and lags between the plotted pulse events and EF, POC flux, DA, and SCOC. P-value ranges for lagged correlations are noted. The duration of pulses is illustrated as a horizontal line above the associated lags and P-value symbols. Transparent gray shading has been added to illustrate the sequence of lags between variables. Numbered POC flux peaks (B) refer to events analyzed from surface to sea floor.

Fig. 4.

POC flux pulse event carbon content and lags. (A) Carbon content and time lags related to specific POC flux pulse events. (B) Average, minimum, and maximum carbon content and time lags from EF to POC flux pulse events to SCOC. For both panels, circle size represents carbon content delivered during the POC flux pulse, along with related EF and SCOC for the same time window shifted by the peak lagged correlation, and line format denotes the P-value range of the peak lagged correlation.

Fig. 5.

Performance of Martin POC flux model estimates for Station M from 1989 to 2017 compared with sediment trap measures. (A) Full time series. (B) Model over/underestimate of POC flux on an annual scale for years with >50% sampling coverage: (POC flux_Martin − POC flux_trap)/POC flux_trap 100.