High-resolution iridium, δ13C, δ18O, foraminifera and nannofossil profiles across the latest Paleocene benthic extinction event at Zumaya, Spain

Abstract

In the expanded upper Paleocene-lower Eocene section (≈ 30 m of Zone P5 sediments) at Zumaya, northern Spain, the highest occurrence of many late Paleocene deep-sea benthic foraminifera species (≈ 40% extinction), coincides with a transition from marl to calcite-free clay. Our high-resolution studies (chemical elements, δ 13 C, δ 18 O, calcareous nannofossils, planktic and benthic foraminifera) show that below the marl-clay transition there is a 40–50 cm thick interval (corresponding to 10–20 kyr) containing a detailed record of a gradual succession of faunal and geochemical events culminating in the benthic extinctions. Planktic foraminiferal and nannofossil changes (e.g., the onset of demise in Fasciculithus genus) occur a few meters below the marl-clay transition. In the limestone 50 cm below the base of the clay, a prominent glauconite maximum indicates that sea-floor oxygenation suddenly decreased. Glauconite continues to be common until the onset of clay deposition. A whole-rock negative δ 13 C shift (1.6‰), most likely reflecting an original sea-water trend, is gradually developed over the 40 cm of greenish brown marls immediately below the clay. At the base of these marls there is a small, significant iridium anomaly of 133 ppt Ir compared with an average background of 38 ppt. In the marls the demise of the Fasciculithus species accelerates, Gavelinella beccariiformis becomes extinct, and the abundance of Acarinina species begins to increase. The superjacent 4 m of clay is devoid of original calcite in its lower part and has a low calcareous content higher up. At calcareous levels in the clay an unusual planktic foraminifera fauna occurs, dominated by Acarinina species. When marl deposition returns, δ 13 C gradually increases and then stabilizes at values about 0.5‰ lower than before the isotopic excursion. The δ 13 C excursion spans in total 5 m, probably corresponding to 200–400 kyr. The fasciculiths disappear shortly after the stabilization of δ 13 C. Here we also present a whole-rock δ 13 C profile through the entire Paleocene section at Zumaya. The profile is very similar to previous profiles registered in well preserved deep-sea material, suggesting that whole-rock δ 13 C at Zumaya can be used for correlation. The origin of the Ir anomaly ≈ 40 cm below the benthic extinction event is enigmatic. It may be impact-related; however, the clearly gradual sequence of environmental events in the 50 cm below the dissolution clay, are more readily reconciled with changing oceanic circulation and palaeogeographic change, possibly in connection with volcanism, than with an instantaneous impact. The small Ir anomaly may reflect such volcanism, sea-water precipitates, or a small impact event, possibly unrelated to the palaeoenvironmental change.