Shelf erosion and submarine river canyons: implications for deep-sea oxygenation and ocean productivity during glaciation

I Tsandev,C P Slomp,C Rabouille,P Van Cappellen

doi:10.5194/bg-7-1973-2010

Abstract

Abstract. The areal exposure of continental shelves during glacial sea level lowering enhanced the transfer of erodible reactive organic matter to the open ocean. Sea level fall also activated submarine canyons thereby allowing large rivers to deposit their particulate load, via gravity flows, directly in the deep-sea. Here, we analyze the effects of shelf erosion and particulate matter re-routing to the open ocean during interglacial to glacial transitions, using a coupled model of the marine phosphorus, organic carbon and oxygen cycles. The results indicate that shelf erosion and submarine canyon formation may significantly lower deep-sea oxygen levels, by up to 25%, during sea level low stands, mainly due to the supply of new material from the shelves, and to a lesser extent due to particulate organic matter bypassing the coastal zone. Our simulations imply that deep-sea oxygen levels can drop significantly if eroded shelf material is deposited to the seafloor. Thus the glacial ocean's oxygen content could have been significantly lower than during interglacial stages. Primary production, organic carbon burial and dissolved phosphorus inventories are all affected by the erosion and rerouting mechanisms. However, re-routing of the continental and eroded shelf material to the deep-sea has the effect of decoupling deep-sea oxygen demand from primary productivity in the open ocean. P burial is also not affected showing a disconnection between the biogeochemical cycles in the water column and the P burial record.

Highlights

The world ocean was a different environment during the glacial periods of the Pleistocene than it is today
The distal coastal zone is already significantly decreased during glacial times due to receding sea level; bypassing an already shrinking sink has a small effect on open ocean dynamics
When we examine the temporal variation in deep-sea oxygen throughout the glacial-interglacial transition (Fig. 3) we see that the oxygen levels under a shelf erosion flux deviate from those of the glacial-interglacial scenario of Tsandev et al (2008)

Summary

Introduction

The world ocean was a different environment during the glacial periods of the Pleistocene than it is today. Trends in authigenic uranium (U) and sedimentary molybdenum (Mo) concentrations point to decreased bottom-water oxygen throughout the Atlantic and in the Pacific, coinciding with glacial periods (Francois et al, 1997; Thomson et al, 1990; Bertine and Turekian, 1973; Mangini et al, 2001; Dean et al, 1997). We explore two additional mechanisms that may, in combination with the climatic forcings studied in Tsandev et al (2008), help explain the observed variations in deep-sea oxygen levels and ocean primary production during glacialinterglacial cycles. In particular we consider two processes not generally included in biogeochemical models for the glacial ocean: the transfer of suspended particulate matter via submarine canyons (“the river canyon hypothesis”) and the erosion of recently deposited sediment from shelves exposed by sea level lowering (“the shelf nutrient hypothesis”)

The river canyon hypothesis

The shelf nutrient hypothesis

Model description and implementation of new mechanisms

Results and discussion

Conclusion and implications