Fig. 2: Relationship of δ238UCAU and U concentrations.
From: Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes
Note that U concentrations are expressed as Ca/U (inverted relative to Fig. 1) in order to emphasize the samples showing U enrichment (lower Ca/U) coupled to high δ238UCAU at Sites 690 (red circles) and 401 (blue circles). These samples demonstrate a near-linear relationship when Ca/U < 8 mol/µmol, which are consistent with a Δ238Uanox of up to ~+0.6‰ relative to inferred δ238Usw. Data from Site 865 (black circles) and pre-PETM data from Site 401 that plot along the flat array of Ca/U likely record δ238Usw. δ238UCAU values below inferred δ238Usw at Site 690 could be caused by the adsorption of isotopically light U directly to calcite or repartitioned during organic matter re-mineralization. Error bars for δ238UCAU represent the external reproducibility of the NIST-SRM 1d limestone standard (±0.06‰, 2 SD).
