Deglacial diatom productivity and surface ocean properties over the Bermuda Rise, northeast Sargasso Sea

Abstract

Diatom assemblages document surface hydrographic changes over the Bermuda Rise. Between 19.2 and 14.5 ka, subtropical diatom species and Chaetoceros resting spores dominate the flora, as in North Atlantic productive regions today. From 16.9 to 14.6 ka, brackish and fresh water diatoms are common and their contribution is generally coupled with total diatom abundance. This same interval also contains rare grains of ice‐rafted debris. Coupling between those proxies suggests that successive discharge of icebergs might have stimulated productivity during Heinrich event 1 (H1). Iceberg migration to the subtropics likely created an isolated environment involving turbulent mixing, upwelled water, and nutrient‐rich meltwater, supporting diatom productivity in an otherwise oligotrophic setting. In addition, the occurrence of mode water eddies likely brought silica‐rich waters of Southern Ocean origin to the euphotic zone. The persistence of lower‐salinity surface water beyond the last ice rafting suggests continued injection of fresh water by cold‐core rings and advection around the subtropical gyre. These results indicate that opal productivity may have biased estimates of meridional overturning based on 231Pa/230Th ratios in Bermuda Rise sediments during H1.

Silicon isotopes indicate enhanced carbon export efficiency in the North Atlantic during deglaciation

“…36) or from the convergence of frontal systems. Introduction of meltwater lenses during icerafting events from the North are consistent with records of smaller diatoms 7 and the deposition of ice-rafted debris at the Bermuda Rise 9 , and the onset of changes in the planktonic foraminiferal d 18 O record from HU89038-PC8 (Fig. 3).…”

“…18). Although there is evidence for increased diatom opal burial in the oligotrophic North Atlantic during Stadials 7 , there has been, as of yet, no quantified study of low-latitude silicon cycling during millennial-scale climatic change and its impact on carbon cycling.…”

“…events of the deglacial 7,8 , which would have had a significant impact on carbon cycling in the otherwise oligotrophic gyre. During the last glacial and deglaciation, abrupt millennial-scale Heinrich Stadials (HS) associated with iceberg-rafting events in the North Atlantic led to widespread and rapid reorganization of ocean circulation, which in turn resulted in shifts in heat transport, atmospheric pCO 2 and the supply of southern component water and nutrients to northern latitudes 9 .…”

Constraining circulation changes through the last deglaciation with deep-sea coral radiocarbon and sedimentary 231Pa/230Th

“…Heinrich events are typically identified in North Atlantic sediment cores as layers of coarse-grained lithogenic fragments most likely transported by icebergs, otherwise known as "ice-rafted debris" [Heinrich, 1988;Hemming, 2004] Th values during H1 (and also during Heinrich Events 2 and 3) correspond with peaks in diatom abundance. Likewise, Gil et al [2009] found that the high 231 Pa/ 230 Th values during H1 in OCE326-GGC5 nearly coincide with an interval of higher diatom content in a nearby core, concluding that opal productivity may have biased earlier interpretation of these values in terms of MOC.…”

“…These results appear inconsistent with 231 Th ratios from the LGM are lower than those from H1, implying a more vigorous circulation during the LGM, whereas the apparent ventilation ages imply that the H1 circulation was more vigorous. Keigwin and Boyle explained the apparent inconsistency between the two proxy records by hypothesizing that the high 231 Pa/ 230 Th observed during H1 result from high diatom (opal) fluxes, as apparently supported by diatom abundance data reported in subsequent work [Gil et al, 2009;Lippold et al, 2009]. Note that the issue of data consistency may not lie solely in reevaluating 231 Pa/ 230 Th (for a critical discussion of 14 C see, e.g., Wunsch [2003]).…”