ABSTRACT Two late Eocene impact spherule layers are known: the North America microtektite layer (from the Chesapeake Bay crater) and the slightly older clinopyroxene (cpx) spherule layer (from Popigai crater). Positive Ir anomalies occur at 5.61 m and 6.19 m above the base of a late Eocene section at Massignano, Italy. The age difference between the two anomalies is ~65 ± 20 k.y. The older Ir anomaly at 5.61 m appears to be associated with the cpx spherule layer. Although no impact spherules or shocked-mineral grains have been found associated with the upper Ir anomaly at 6.19 m, it has been proposed that it may be from the Chesapeake Bay impact. Comparison with other distal ejecta layers suggests that microtektites, but not shocked-mineral grains, from the Chesapeake Bay crater could have been thrown as far as Massignano. However, their absence neither supports nor disproves the hypothesis that the Ir anomaly at 6.19 m is from the Chesapeake Bay impact. On the other hand, the North American microtektite layer is not associated with an Ir anomaly. Furthermore, the average age difference between the cpx spherule layer and the North American microtektite layer appears to be ~18 ± 11 k.y., which is nearly one quarter the age difference between the two Ir anomalies at Massignano. This indicates that the Ir anomaly at 6.19 m is too young to be from the Chesapeake Bay impact, and thus is most likely not from the Chesapeake Bay impact.

The iron oxidation state and coordination number have been determined by Fe K-edge X-ray absorption–near edge spectroscopy (XANES) for six tektites from the North American tektite strewn field (four bediasites and two georgiaites) and a tektite fragment from the Deep Sea Drilling Project (DSDP) Site 612, which is part of the North American tektite strewn field. All tektite samples display a pre-edge peak, the components of which are consistent with the presence of divalent Fe. Comparison of pre-edge peak data (integrated intensity and centroid energy) with those of Fe model compounds allows us to quantitatively determine the Fe oxidation state and coordination number. The Fe 3+ /(Fe 2+ + Fe 3+ ) ratio is close to 0.05 (±0.03), consistent with tektites from all the other strewn fields. The average Fe coordination number is intermediate between 4 and 5. Accurate determination of this value is hindered by the scatter of pre-edge peak intensity of all the Fe model compounds; [4] Fe/( [4] Fe + [5] Fe) is estimated to be 0.26 ± 0.15. In contrast to North American microtektites, for which iron oxidation state and coordination numbers display significant variations across the North American strewn field, no significant variations in the Fe oxidation state and coordination number were found between the georgiaites, bediasites, and the DSDP 612 tektite fragment. It is remarkable that the tektite fragment from the DSDP 612 site showed no sign of oxidation despite the long burial period in marine sediments.

We evaluated the age of two Upper Eocene impact ejecta layers (North American microtektites linked to the Chesapeake Bay impact structure and clinopyroxene [cpx] spherules from the Popigai crater) and the global effects of the associated impact events. The reported occurrence of cpx spherules from the Popigai impact structure at South Atlantic ODP Site 1090 within the middle of magnetochron C16n.1n yields a magnetochronologic age of 35.4 Ma. We generated high-resolution stable isotope records at Sites 1090, 612 (New Jersey slope), and Caribbean core RC9-58 that show: (1) a 0.5‰ δ 13 C decrease in bulk-carbonate at Site 1090 coincident with the Popigai cpx spherule layer, and (2) a 0.4‰–0.5‰ decrease in deep-water benthic foraminiferal δ 13 C values across the Popigai impact ejecta layer at Site 612 and core RC9-58. We conclude that the δ 13 C excursion associated with Popigai was a global event throughout the marine realm that can be correlated to magnetochron C16n.1n. The amplitude of this excursion (~0.5‰) is within the limits of natural variability, suggesting it was caused by a decrease in carbon export productivity, potentially triggered by the impact event(s). North American microtektites associated with the Chesapeake Bay impact occur stratigraphically above the Popigai cpx spherules at Site 612 and core RC9-58. We found no definite evidence of a δ 13 C anomaly associated with the North American microtektite layer, though further studies are warranted. High-resolution bulk-carbonate and benthic foraminiferal δ 18 O records show no global temperature change associated with the cpx spherule or North American microtektite layers.