Foraminiferal response to seasonality modulated by orbital cycles in the Cretaceous mid-latitudes: The Albian record from the Lower Saxony Basin

Abstract

The mid-Cretaceous serves as an excellent laboratory for testing the dynamics of the climate under greenhouse state of the Earth. This study is focused on the cyclicity record of foraminiferal assemblages in the Lower Saxony Basin during the Middle Albian. The short eccentricity, obliquity, and precession cycles are consistently recorded in absolute and relative abundance of most opportunistic benthic and planktic foraminifers. The question is why foraminifers reacted to orbital cycles and what they actually record in the “equable” global climate. It is suggested that seasonality changes modulated by orbital cycles are responsible for the Milankovitch cyclicity records. It is well known that the axial obliquity directly influences seasonal contrasts in mid- and high latitudes. The precession modulated by the eccentricity of the Earth orbit further affects the contrast of seasons. Therefore, seasonal dynamics of precipitation/runoff and storm tracks patterns, and to a lesser extent temperature contrasts, are most likely factors controlling Milankovitch-scale cyclic fluctuations of foraminiferal assemblages and other proxies sensitive to productivity changes. It can be deduced that periods of the highest seasonal contrasts were dominated by opportunistic, abundant taxa, such as benthic Valvulineria–Gyroidinoides group and planktic hedbergellids, which record the strongest obliquity signals. The Lower Saxony and adjacent Boreal basins were possibly controlled by humid climate with relatively dry summers and wet winter seasons. This study confronted with the presented conceptual model and former climate models supports the conclusion that the temperate zone surrounding Tethys was sensitive to seasonal changes modulated by orbital cyclicity. The mid-latitudinal climate of the greenhouse Earth seems to be more dynamic and more seasonal than expected.