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 WT sequences
despite the fact that this section was deposited near the Cretaceous
equator. The KIT 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 Pie, some 300-500 kyr
after the WT event.
delta(13)C values from bulk carbonates show both high-and low-latitude
characteristics. Similarly to low latitudes, there is a 3 parts per
thousand negative delta(13)C excursion at the KIT 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 WT sequences, a negative delta(13)C
shift occurs in the early Danian Zone PIb about 300 kyr after the WT
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 WT 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 delta(13)C pattern combines
ameliorated low-latitude effects with predominantly high-latitude
trends.