The interaction of both volcanism and enhanced global weathering on Osi means that quantifying the magnitude and isolating the extent of the two signals is problematic, since the extent of weathering on seawater chemistry is masked by the inputs from the Caribbean LIP to the global ocean. We can only estimate the Os contribution to seawater chemistry using a mixing model and assumed abundances. If we assume that the average seawater 187Os/188Os prior to the LIP onset was ∼0.8, and use an average Os abundance in seawater of 10 ppq (based on the present-day average; Peucker-Ehrenbrink and Ravizza, 2000), a basalt 187Os/188Os of 0.13 (Meisel et al., 2001) and an average Os abundance, we can evaluate the approximate Os contribution from the Caribbean LIP to the global ocean using a progressive mixing model (Faure, 1986, Eqs. (9.2) and (9.10)). We note that there are no published Os data for the Caribbean LIP. However, basalts can have variable Os abundances (1 to 600 ppt; Martin, 1991; Crocket and Paul, 2008); typical values range from 1 to 30 ppt (e.g., Shirey and Walker, 1998; Allegre et al., 1999; Dale et al., 2008). Using an Os abundance for a basalt of 30 ppt would require 75% Os contribution from the LIP to yield the least radiogenic Osi observed at all locations. Considerably less Os input from the LIP (25%) is needed if the LIP basalts possess higher Os abundances (100 ppt) and if the Os contribution to seawater also occurred through the addition of gas known to be enriched 20 times that of the basalt (e.g., Yudovskya et al., 2008).
seawater
187Os/188Os
∼0.8
seawater
Os abundance
10 ppq
ppq
basalt
187Os/188Os
0.13
basalts
Os abundances
1 to 600 ppt
ppt
basalts
Os abundances
1 to 30 ppt
ppt
basalt
Os abundance
30 ppt
ppt
Os contribution
from the LIP
75%
%
Os input
from the LIP
25%
%
LIP basalts
Os abundances
100 ppt
ppt
gas
20 times
times