To understand the climate dynamics of hypothesized past greenhouse intervals, it is essential to constrain tropical sea-surface temperatures (SST), yet existing proxy records give conflicting results. Here we present the first Mg/Ca-based study of pre-Quaternary SST and investigate early Paleogene (late Paleocene through late middle Eocene; 58.6-39.8 Ma) tropical temperatures, using planktonic foraminifera belonging to the genus Morozovella from Ocean Drilling Program Site 865 on Allison Guyot (western central equatorial Pacific Ocean). Calcification temperatures similar to or warmer than modern tropical SST are calculated using a range of assumptions regarding diagenesis, temperature calibration, and seawater Mg/Ca. Long-term warming is observed into the early Eocene (54.8-49.0 Ma), with peak SST between 51 and 48 Ma and rapid cooling of 4°C beginning at 48 Ma. These findings are inconsistent with the d18O-based SST previously estimated for this site.

Sediment depth is given in mbsf. Mg/Ca ratios have been adjusted using the average species offset from M. aragonensis. The offset is +0.16 mmol/mol for M. lehneri, -0.06 mmol/mol for M. spinulosa, -0.20 for M. subbotinae, and -0.08 for M. velazcoensis. bSr/Ca ratios have been adjusted using the average species offset from M. aragonensis. No offset is observed between M. spinulosa and M. aragonensis. The offset is +0.04 mmol/mol for M. lehneri, -0.03 for M. subbotinae, and -0.03 for M. velazcoensis. Samples without normalization calculation values have been excluded from discussion because of high Fe/Ca (>0.10 mmol/mol) and Fe/Mg (>0.02 mol/mol) ratios.

Supplement to: Tripati, Aradhna K; Delaney, Margaret Lois; Zachos, James C; Anderson, Linda Davis; Kelly, Daniel Clay; Elderfield, Henry (2003): Tropical sea-surface temperature reconstruction for the early Paleogene using Mg/Ca ratios of planktonic foraminifera. Paleoceanography, 18(4), 1101