Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales

Abstract

Abstract. A more than two-decadal sediment trap record from the Eastern Boundary Upwelling Ecosystem (EBUE) off Cape Blanc, Mauritania, is analysed with respect to deep ocean mass fluxes, flux components and their variability on seasonal to decadal timescales. The total mass flux revealed interannual fluctuations which were superimposed by fluctuations on decadal timescales. High winter fluxes of biogenic silica (BSi), used as a measure of marine production (mostly by diatoms) largely correspond to a positive North Atlantic Oscillation (NAO) index (December-March). However, this relationship is weak. The highest positive BSi anomaly was in winter 2004-2005 when the NAO was in a neutral state. More episodic BSi sedimentation events occurred in several summer seasons between 2001 and 2005, when the previous winter NAO was neutral or even negative. We suggest that distinct dust outbreaks and deposition in the surface ocean in winter and occasionally in summer/autumn enhanced particle sedimentation and carbon export on short timescales via the ballasting effect. Episodic perturbations of the marine carbon cycle by dust outbreaks (e.g. in 2005) might have weakened the relationships between fluxes and largescale climatic oscillations. As phytoplankton biomass is high throughout the year, any dry (in winter) or wet (in summer) deposition of fine-grained dust particles is assumed to enhance the efficiency of the biological pump by incorporating dust into dense and fast settling organic-rich aggregates. A good correspondence between BSi and dust fluxes was observed for the dusty year 2005, following a period of rather dry conditions in the Sahara/Sahel region. Large changes of all bulk fluxes occurred during the strongest El Niño-Southern Oscillation (ENSO) in 1997-1999 where low fluxes were obtained for almost 1 year during the warm El Niño and high fluxes in the following cold La Niña phase. For decadal timescales, Bakun (1990) suggested an intensification of coastal upwelling due to increased winds ("Bakun upwelling intensification hypothesis"; Cropper et al., 2014) and global climate change. We did not observe an increase of any flux component off Cape Blanc during the past 2 and a half decades which might support this. Furthermore, fluxes of mineral dust did not show any positive or negative trends over time which might suggest enhanced desertification or "Saharan greening" during the last few decades.

Coccolithophore fluxes in the open tropical North Atlantic: influence of thermocline depth, Amazon water, and Saharan dust

“…A longer distance to the dust source in Africa at M4 compared to M2 may have also contributed to increasing the bioavailability of dust-driven nutrients further west (see Stuut and Prins, 2014). This could also help explain the lack of clear evidence for dust fertilization in open-ocean regions west of Africa (see Fischer et al, 2016;Neuer et al, 2004) since aerosols sinking in these areas would not be fine and chemically processed enough to act as fertilizers.…”

“…To confirm whether the pulsed flux maxima of opportunistic species presented in our study truly reflected the response of living coccolithophores thriving in the overlying photic layer to such a combination of factors (ecological signal) or resulted from enhanced particle transfer efficiency (e.g. ballasting by dust; faecal pellet production by zooplankton grazers; Armstrong et al, 2002;Ziveri et al, 2007;Fischer and Karakas, 2009;Fischer et al, 2016), a comparison between settling coccolith assemblages and the living coccolithophore communities as well as with in situ atmospheric-oceanographic observations would be required.…”

Tropical Rains Controlling Deposition of Saharan Dust Across the North Atlantic Ocean

“…Reported seasonality of dust concentrations and deposition is different on either side of the Atlantic, peaking in winter in the east (Bory et al, 2002;Fischer et al, 2016;Fomba et al, 2014;Neuer et al, 2004;Ratmeyer et al, 1999;Skonieczny et al, 2013), and in summer in the west (Jickells et al, 1998;Prospero et al, 2014). These differences can be related to seasonal atmospheric dynamics like the altitude at which the dust is transported, with dust at lower altitudes deposited close to the source in the east, while dust transported at higher altitudes can reach the remote western Atlantic ( Van der Does et al, 2016;Yu et al, 2019).…”

No Correlation Between Atmospheric Dust and Surface Ocean Chlorophyll‐a in the Oligotrophic Gulf of Aqaba, Northern Red Sea

“…In open ocean oligotrophic settings, the partial dissolution of mineral dust particles in surface waters can strongly modulate marine biogeochemical cycles by increasing primary productivity (e.g., Pabortsava et al, 2017) through the supply of limiting nutrients, such as Fe, N, P, Co, Zn, and others, and therefore drive the uptake of atmospheric carbon into the oceans. In addition, mineral dust particles can act as ballast, increasing the sinking rate of organic carbon in marine snow and other aggregates, especially close to continental margins (Fischer et al, 2016;Francois et al, 2002). Indeed, atmospheric iron fertilization in currently iron-limited ecosystems such as the subantarctic Southern Ocean has been argued to have enhanced export productivity and contributed to lower atmospheric CO 2 levels during past glacials (e.g., Martin, 1990;Martínez-García et al, 2014).…”