Pollen from the Deep-Sea: A Breakthrough in the Mystery of the Ice Ages

Abstract

Pollen from deep-sea sedimentary sequences provides an integrated regional reconstruction of vegetation and climate (temperature, precipitation, and seasonality) on the adjacent continent. More importantly, the direct correlation of pollen, marine and ice indicators allows comparison of the atmospheric climatic changes that have affected the continent with the response of the Earth's other reservoirs, i.e., the oceans and cryosphere, without any chronological uncertainty. The study of long continuous pollen records from the European margin has revealed a changing and complex interplay between European climate, North Atlantic sea surface temperatures (SSTs), ice growth and decay, and high-and low-latitude forcing at orbital and millennial timescales. These records have shown that the amplitude of the last five terrestrial interglacials was similar above 40 • N, while below 40 • N their magnitude differed due to precessionmodulated changes in seasonality and, particularly, winter precipitation. These records also showed that vegetation response was in dynamic equilibrium with rapid climate changes such as the Dangaard-Oeschger (D-O) cycles and Heinrich events, similar in magnitude and velocity to the ongoing global warming. However, the magnitude of the millennial-scale warming events of the last glacial period was regionally-specific. Precession seems to have imprinted regions below 40 • N while obliquity, which controls average annual temperature, probably mediated the impact of D-O warming events above 40 • N. A decoupling between high-and low-latitude climate was also observed within last glacial warm (Greenland interstadials) and cold phases (Greenland stadials). The synchronous response of western European vegetation/climate and eastern North Atlantic SSTs to D-O cycles was not a pervasive feature throughout the Quaternary. During periods of ice growth such as MIS 5a/4, MIS 11c/b and MIS 19c/b, repeated millennial-scale cold-air/warm-sea decoupling events occurred on the European margin superimposed to a long-term air-sea decoupling trend. Strong air-sea thermal contrasts promoted the production of water vapor that was then transported northward by the Frontiers in Plant Science | www.frontiersin.org 1 January 2018 | Volume 9 | Article 38Sánchez Goñi et al.Pollen and Ice Ages westerlies and fed ice sheets. This interaction between long-term and shorter timescale climatic variability may have amplified insolation decreases and thus explain the Ice Ages. This hypothesis should be tested by the integration of stochastic processes in Earth models of intermediate complexity.

Monsoonal Forcing of European Ice‐Sheet Dynamics During the Late Quaternary

“…We argue that this temporal coincidence is a direct consequence of warm‐water advection toward southwest Europe caused by the enhanced entrainment of MOW into the North Atlantic during precession maximum (insolation minimum) conditions. The accumulation of warm surface waters in the eastern North Atlantic led to enhanced formation of atmospheric moisture (Sánchez Goñi et al, ; Figure a) that was advected northeastward into the continent as documented by central European paleosol records (Schirmer, ; Zeeden et al, ). The atmospheric moisture transport was further enhanced by the reorganization of the North Atlantic jet stream during glacial periods (Luetscher et al, ), with a southward displaced and intensified jet stream as a response to the growth of the Laurentide and European Ice Sheets (Löfverström & Lora, ).…”

“…Hence, our proposed mechanism should primarily affect the SW part of the EIS. Nonetheless, based on the notion that moisture from the Iberian Margin is transported as far north as the Arctic (Sánchez Goñi et al (), MOW‐driven moisture accumulation may also have played a role beyond central Europe.…”

“…Here its meltwaters have the potential to disrupt deepwater formation, thereby causing a weakening of the meridional overturning circulation (Hodell et al, ; Toucanne et al, ). The rapid regrowth of the Late Quaternary glacial EIS after intermittent phases of destabilization during times of increasing summer insolation required an enhanced moisture supply (Sánchez Goñi et al, ). However, the mechanism fuelling moisture advection to the EIS during full glacial conditions has remained largely elusive.…”

Testing the Tropical Trigger Hypothesis of Abrupt Climate Variability

“…We need accurate knowledge on natural climate variability to comprehend how human activities impact climate. We are yet to understand the natural triggers of rapid climate variability and how regional abrupt climate shifts can modify the global climate (Hodell and Channell, 2016;Sánchez Goñi et al, 2018).…”

Time Machine Biology: Cross-Timescale Integration of Ecology, Evolution, and Oceanography

“…Many microfossils represent parts of larger organisms, such as the scales and teeth of fish (Field et al, 2009;Sibert et al, 2017) and shark denticles (Dillon et al, 2017), referred to as ichthyoliths. Although pollen and spores have terrestrial origins, they can also be preserved in both marine and freshwater sediments (Sánchez Goñi et al, 2018).…”