The Cretaceous/Tertiary boundary event in Ecuador: reduced biotic effects due to eastern boundary current setting

Abstract

A multidisciplinary study of a new Cretaceous-Tertiary (K/T) boundary section near Guayaquil, Ecuador, reveals an unusually cool water, low diversity planktic foraminiferal fauna and a high diversity radiolarian fauna similar to those found in southern high-latitude K/T sequences despite the fact that this section was deposited near the Cretaceous equator. The K/T boundary is located by planktic foraminifera within a narrow interval bounded by last appearances of tropical Cretaceous species and first appearances of Tertiary species including Parvularugoglobigerina eugubina. As in southern high latitudes, there is no major mass extinction of either planktic foraminifera or radiolarians at this level. A major radiolarian faunal discontinuity occurs some 6 m higher in the section within foraminiferal Zone Plc, some 300–500 kyr after the K/T event. δ 13C values from bulk carbonates show both high- and low-latitude characteristics. Similarly to low latitudes, there is a 3%. negative δ 13C excursion at the K/T boundary which is generally interpreted as a major decrease in primary productivity. But unlike the low latitudes, recovery occurs within a few thousand years, as compared with 300–500 kyr, and suggests rapid nutrient influx from the Antarctic region via a current similar to the Humboldt current today. Similarly to high-latitude K/T sequences, a negative δ 13C shift occurs in the early Danian Zone Plb about 300 kyr after the K/T boundary. Sedimentologic and mineralogic data indicate a late Maastrichtian with relatively low biogenic quartz and high carbonate followed by increasing biogenic quartz (>50%) and decreasing carbonate (<5%) during the early Danian. This suggests intensified atmospheric and oceanic circulation and upwelling off Ecuador during the early Danian. The K/T transition is marked by increased volcanic activity, continental erosion and terrigenous influx, but this also occurs in the early Danian Zone at the P1a P1b zonal transition and is thus not unique to the K/T boundary. We suggest that the catastrophic biotic effects normally observed at the K/T boundary in low latitudes are greatly reduced or absent in the eastern equatorial Pacific because this region was dominated, then as now, by upwelling and current transport of nutrient-rich waters from the Antarctic Ocean. As a result, the biotic patterns are characteristic of southern high latitudes, whereas the δ 13C pattern combines ameliorated low-latitude effects with predominantly high-latitude trends.