North Atlantic marine biogenic silica accumulation through the early to middle Paleogene: implications for ocean circulation and silicate weathering feedback

Abstract

Abstract. The Paleogene history of biogenic opal accumulation in the North Atlantic provides insight into both the evolution of deepwater circulation in the Atlantic basin and weathering responses to major climate shifts. However, existing records are compromised by low temporal resolution and/or stratigraphic discontinuities. In order to address this problem, we present a multi-site, high-resolution record of biogenic silica (bioSiO2) accumulation from Blake Nose (ODP Leg 171B, western North Atlantic) spanning the early Paleocene to late Eocene time interval (∼65–34 Ma). This record represents the longest single-locality history of marine bioSiO2 burial compiled to date and offers a unique perspective into changes in bioSiO2 fluxes through the early to middle Paleogene extreme greenhouse interval and the subsequent period of long-term cooling. Blake Nose bioSiO2 fluxes display prominent fluctuations that we attribute to variations in sub-thermocline nutrient supply via cyclonic eddies associated with the Gulf Stream. Following elevated and pulsed bioSiO2 accumulation through the Paleocene to early Eocene greenhouse interval, a prolonged interval of markedly elevated bioSiO2 flux in the middle Eocene between ∼46 and 42 Ma is proposed to reflect nutrient enrichment at Blake Nose due to invigorated overturning circulation following an early onset of Northern Component Water export from the Norwegian–Greenland Sea at ∼49 Ma. Reduced bioSiO2 flux in the North Atlantic, in combination with increased bioSiO2 flux documented in existing records from the equatorial Pacific between ∼42 and 38 Ma, is interpreted to indicate diminished nutrient supply and reduced biosiliceous productivity at Blake Nose in response to weakening of the overturning circulation. Subsequently, in the late Eocene, a deepwater circulation regime favoring limited bioSiO2 burial in the Atlantic and enhanced bioSiO2 burial in the Pacific was established after ∼38 Ma, likely in conjunction with re-invigoration of deepwater export from the North Atlantic. We also observe that Blake Nose bioSiO2 fluxes through the middle Eocene cooling interval (∼48 to 34 Ma) are similar to or higher than background fluxes throughout the late Paleocene–early Eocene interval (∼65 to 48 Ma) of intense greenhouse warmth. This observation is consistent with a temporally variable rather than constant silicate weathering feedback strength model for the Paleogene, which would instead predict that marine bioSiO2 burial should peak during periods of extreme warming.

Surface Ocean Cooling in the Eocene North Atlantic Coincides With Declining Atmospheric CO2

“…There is growing geochemical and sedimentological evidence placing the initial onset of NCW between ∼49 and 47 Ma, coincident with changes in zonal temperature gradients between the eastern and western North Atlantic. Evidence for onset of NCW between ∼49 and 47 Ma includes development of contourite drifts in the western North Atlantic (Boyle et al., 2017), changes in biosiliceous sedimentation (Witkowski et al., 2021) and a collapse in δ 13 C gradients between the North and South Atlantic (Hohbein et al., 2012). These changes would also influence local hydrography within the eastern North Atlantic and could exert an additional control on δ 18 O sw values at DSDP Site 401.…”

“…The onset of NCW has been attributed to gateway reorganisation, specifically deepening of the Greenland‐Scotland Ridge (GSR) (Boyle et al., 2017; Hohbein et al., 2012; Vahlenkamp et al., 2018), although other mechanisms have been proposed such as isolation of the Arctic Ocean (Z. Zhang et al., 2011) or restriction of the Tethys Ocean (Roberts et al., 2009). The onset of NCW is followed by a period of weaker overturning (∼42–38 Ma) (Witkowski et al., 2021), before re‐invigoration of NCW during the late Eocene (∼38 Ma) (Coxall et al., 2018) or Eocene‐Oligocene transition (EOT; ∼34 Ma) (Hutchinson et al., 2019). The establishment of NCW can transport additional heat into the eastern North Atlantic (Vahlenkamp et al., 2018), potentially muting any long‐term cooling trend in this region and has been invoked to explain stable temperatures in the eastern North Atlantic during the middle Eocene (Bornemann et al., 2016).…”

Surface ocean cooling in the Eocene North Atlantic coincides with declining atmospheric CO2

“…There is growing geochemical and sedimentological evidence placing the initial onset of NCW between ∼49 and 47 Ma, coincident with changes in zonal temperature gradients between the eastern and western North Atlantic. Evidence for onset of NCW between ∼49 and 47 Ma includes development of contourite drifts in the western North Atlantic (Boyle et al, 2017), changes in biosiliceous sedimentation (Witkowski et al, 2021) and a collapse in δ 13 C gradients between the North and South Atlantic (Hohbein et al, 2012). These changes would also influence local hydrography within the eastern North Atlantic and could exert an additional control on δ 18 O sw values at DSDP Site 401.…”

“…Greenland-Scotland Ridge (GSR) (Boyle et al, 2017;Hohbein et al, 2012;Vahlenkamp et al, 2018), although other mechanisms have been proposed such as isolation of the Arctic Ocean (Z. or restriction of the Tethys Ocean (Roberts et al, 2009). The onset of NCW is followed by a period of weaker overturning (∼42-38 Ma) (Witkowski et al, 2021), before re-invigoration of NCW during the late Eocene (∼38 Ma) (Coxall et al, 2018) or Eocene-Oligocene transition (EOT; ∼34 Ma) (Hutchinson et al, 2019). The establishment of NCW can transport additional heat into the eastern North Atlantic (Vahlenkamp et al, 2018), potentially muting any long-term cooling trend in this region and has been invoked to explain stable temperatures in the eastern North Atlantic during the middle Eocene (Bornemann et al, 2016).…”