Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous

Abstract

ABSTRACT. There were three negative seawater strontium-isotope excursions (shifts to lower 87 Sr/ 86Sr values) during the Jurassic and Cretaceous that were of relatively short duration (5-13 my) and showed a relatively quick recovery to pre-excursion 87 Sr/ 86 Sr ratios. These excursions occurred in the Pliensbachian-Toarcian (Early Jurassic), Aptian-Albian, and Cenomanian-Santonian (Early and Late Cretaceous respectively). Each excursion coincided closely in time with an Oceanic Anoxic Event (OAE) marked by sediments unusually rich in organic carbon. The Jurassic OAE occurred at the end of the strontium-isotope excursion, whereas the two Cretaceous OAEs occurred at the onset of the accompanying strontium-isotope excursions.The possible causes of these excursions were evaluated by successively examining the changes in the riverine strontium fluxes, riverine 87 Sr/ 86 Sr ratios, or hydrothermal strontium fluxes required to produce each excursion. A range of seawater strontium budgets was used to encompass the uncertainties in modern and ancient cycles. To produce the excursions, we calculate that the riverine strontium fluxes would have had to decrease by 6 to 15 percent or the fluvial 87 Sr/ 86 Sr ratios by 0.00019 to 0.00046. The uncertainties largely stem from the assumed magnitude of the hydrothermal strontium flux at the onset of each excursion. Alternatively, increases in sea-floor hydrothermal activity of 7 to 104 percent could also have produced the strontium-isotope excursions. This large range is due mostly to uncertainties in the relative flux of strontium from axial high-temperature hydrothermal systems and low-temperature off-axis systems. Only a small portion of this range stems from uncertainties in the riverine strontium terms.The possible causes of the excursions were further evaluated by examining several geologic factors that could have affected riverine strontium, including climate change, sealevel, and the eruption of flood basalts. We conclude that neither variations in riverine strontium fluxes nor in 87 Sr/ 86 Sr ratios is the likely cause of the strontiumisotope excursions. The most probable explanation is increased rates of hydrothermal activity related to increased ocean-crust production at the mid-ocean ridges.The close correlation in time between the strontium-isotope excursions and the major Oceanic Anoxic Events (OAEs) is compatible with a causal linkage. We propose that increased ocean-crust production led to enhanced CO 2 outgassing and global warming, which in turn led to several processes that acted to make surface ocean waters more productive. However, because OAEs did not occur throughout the proposed periods of enhanced hydrothermal activity, it appears that these processes only preconditioned the oceans for the OAEs: sealevel rise may have been the final trigger. This model explains why all three OAEs did not occur at the same time relative to the onset of excess hydrothermal activity and why OAEs are not associated with every sealevel rise documented in the stratigraphic re...

Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)

“…According to Jones & Jenkyns (2001), this negative excursion in strontium isotopes can be interpreted as the result of increased ocean-ridge hydrothermal activity. Os isotopes, studied by Bottini et al (2012) and Tejada et al (2009) …”

“…Enhanced weathering rates have been also proposed on the basis of a negative shift in  44/42 Ca (Blättler et al, 2011), although calcium isotopes can potentially be affected by ocean acidification, temperature changes and/or calcification rate (Fantle & Tipper, 2014). This continental weathering pulse is not unequivocally captured by the marine strontium-isotope curve, although some relatively radiogenic values do overlap with the OAE 1a record (Jones & Jenkyns, 2001). Essentially, however, the weathering pulse was probably too short or too small to offset the overwhelming contribution of non-radiogenic strontium delivered by enhanced hydrothermal/magmatic activity associated with construction (Bralower et al, 1997;Jones & Jenkyns, 2001) and/or destruction of the LIP (Allègre et al, 2010;Pogge von Strandmann et al, 2013).…”

“…This continental weathering pulse is not unequivocally captured by the marine strontium-isotope curve, although some relatively radiogenic values do overlap with the OAE 1a record (Jones & Jenkyns, 2001). Essentially, however, the weathering pulse was probably too short or too small to offset the overwhelming contribution of non-radiogenic strontium delivered by enhanced hydrothermal/magmatic activity associated with construction (Bralower et al, 1997;Jones & Jenkyns, 2001) and/or destruction of the LIP (Allègre et al, 2010;Pogge von Strandmann et al, 2013). In a more qualitative way, sedimentological studies also suggest enhanced weathering at the onset of OAE 1a: Stein et al (2012), for instance, proposed at least a local increase in continental chemical weathering on the basis of a rise in the relative abundance of the clay mineral kaolinite in south-east France.…”

“…The early Aptian OAE 1a or Selli event (~120 Ma) has been much studied in terms of carbon-, calcium-, osmiumand strontium-isotope stratigraphy: Blättler et al (2011); Bottini et al (2012); Erba et al (2010); ; ; Jones & Jenkyns (2001); Keller et al (2011); Méhay et al (2009) ;Menegatti et al (1998); Tejada et al (2009);van Breugel et al (2007); Wissler et al (2004). This event is conventionally interpreted as triggered by the massive outgassing of the Ontong Java, Manihiki and Hikurangi submarine large igneous province (LIP; Larson & Erba, 1999;Méhay et al, 2009;Tarduno et al, 1991Tejada et al, 2009, which introduced large quantities of isotopically light carbon into the atmosphere-ocean system, producing a stepwise decline in  13 C and lowering the pH of the oceans.…”

Carbon cycle history through the Jurassic–Cretaceous boundary: A new global δ13C stack

“…and consequent greater access of seawater to hotter, fresher material at the ridge axis (Jones and Jenkyns, 2001). …”