Seasonal variation in the Mg/Ca ratio and δ18O of the planktonic foraminifera Globigerina bulloides: Results from the Gulf of Tehuantepec, Eastern Equatorial North Pacific

Mg/Ca and δ18O has been measured in the surface dwelling planktonic foraminiferal species Globigerina bulloides from weekly sediment trap samples collected from two depths (460 and 560m) in the Gulf of Tehuantepec (15°38.826 N, 95°16.905 W) between February 2006 and June 2007. The Gulf of Tehuantepec is a region of intense upwelling and undergoes significant seasonal changes in sea surface temperature (SST) (~ 26° - 30°C) and salinity (~ 33.3 - 34.5). This is an ideal setting to study the effects of temperature and salinity on δ18O by comparing it to the temperature dependent, but salinity independent Mg/Ca ratio. Our results show that Mg/Ca and δ18O are negatively correlated (r2 =- 0.61, p < 0.001) with most of the scatter occurring in the region of low Mg/Ca and high δ18O values. A comparison of the Mg/Ca values with monthly temperature measurements yields a strong positive correlation (r2 = 0.91, p = 0.003), while only a weak negative correlation exists between the δ18O and temperature (r2 = -0.23, p = 0.021). The Mg/Ca data were also compared to monthly salinity measurements and showed a weak negative correlation (r2 = - 0.39, p = 0.186), while the δ18O data show no significant correlation with salinity. Temperatures calculated from the Mg/Ca data using the Elderfield and Ganssen (2000) equation range from 24° - 30°C and thus agree fairly well with the measured temperatures. Conversely, temperatures calculated from δ18O using the Bemis et al. (1998) equation ranged from 19° - 30°C. Weekly satellite SST measurements are being acquired and will allow us to better evaluate the relationship between temperature and both Mg/Ca and δ18O.