There were three negative seawater strontium-isotope excursions (shifts to
lower Sr-87/Sr-86 values) during the Jurassic and Cretaceous that were of r
elatively short duration (5-13 my) and showed a relatively quick recovery t
o pre-excursion Sr-87/Sr-86 ratios. These excursions occurred in the Pliens
bachian-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 ri
ch in organic carbon. The Jurassic OAE occurred at the end of the strontium
-isotope excursion, whereas the two Cretaceous OAEs occurred at the onset o
f the accompanying strontium-isotope excursions.
The possible causes of these excursions were evaluated by successively exam
ining the changes in the riverine strontium fluxes, riverine Sr-87/Sr-86 ra
tios, or hydrothermal strontium fluxes required to produce each excursion.
A range of seawater strontium budgets was used to encompass the uncertainti
es in modern and ancient cycles. To produce the excursions, we calculate th
at the riverine strontium fluxes would have had to decrease by 6 to 15 perc
ent or the fluvial Sr-87/Sr-86 ratios by 0.00019 to 0.00046. The uncertaint
ies largely stem from the assumed magnitude of the hydrothermal strontiun f
lux at the onset of each excursion. Alternatively, increases in sea-floor h
ydrothermal activity of 7 to 104 percent could also have produced the stron
tium-isotope excursions. This large range is due mostly to uncertainties in
the relative flux of strontium from axial high-temperature hydrothermal sy
stems and low-temperature off-axis systems. Only a small portion of this ra
nge stems from uncertainties in the riverine strontium terms.
The possible causes of the excursions were further evaluated by examining s
everal geologic factors that could have affected riverine strontium, includ
ing climate change, sealevel, and the eruption of flood basalts. We conclud
e that neither variations in riverine strontium fluxes nor in Sr-87/Sr-86 r
atios is the likely cause of the strontium-isotope excursions. The most pro
bable explanation is increased rates of hydrothermal activity related to in
creased 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 COP outga
ssing and global warming, which in turn led to several processes that acted
to make surface ocean waters more productive. However, because OAEs did no
t 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 al
l 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 record.