Piacenzian Environmental Change and the Onset of Cool and Dry Conditions in Tropical South America

• Published in: Paleoceanography and paleoclimatology 2020-11, Vol.35 (11), p.n/a
• Main Authors: Grimmer, Friederike, Dupont, Lydie M., Jung, Gerlinde, Wefer, Gerold
• Format: Article
• Language: English
• Subjects: Amplification; Climate and vegetation; Climate change; Climate models; Coastal climates; Cooling; Drilling; Drying; Ecuador; Environmental changes; Fossil pollen; Fossils; Future climates; Glacial climates; Glacial periods; Glaciation; Glaciology; Global climate; Greenhouse effect; Greenhouse gases; Ice cover; Ice sheets; Interglacial periods; Marine sediments; mid‐Pliocene warm period; Neogene; Northern Hemisphere; Oscillations; paleoclimate; paleovegetation; Palynology; Pleistocene; Pollen; Rain; Rainfall; Sea surface; Sea surface temperature; Sediment samplers; Sediment samples; Sediments; Surface temperature; Tropical climate; Tropical environments; Vegetation; Vegetation changes; Walker circulation; Warm climates; western Andean cordillera
• ISSN: 2572-4517; 2572-4525
• DOI: 10.1029/2020PA004060

The Piacenzian (3.60–2.58 Ma) covers the last stage of the Neogene just before the Earth's climate turned from relatively stable warm conditions to the cooler climate with high amplitude glacial‐interglacial oscillations of the Pleistocene. Even during this period early fluctuations towards cooler conditions occurred, and sea surface temperature (SST) reconstructions show stepwise increasing gradients. The zonal Pacific SST gradient which indicates the strength of the Walker circulation appears to have increased in two steps starting in the Piacenzian. We investigated vegetation and climate change in western equatorial South America under the influence of the Walker circulation and to detect signs for the onset of cooling in the tropics. We studied vegetation changes in western Ecuador using palynological analysis of 88 sediment samples from marine Ocean Drilling Program Site 1239 dated between 3.9 and 2.7 Ma. A general trend towards more open vegetation is observed. The climate changes towards cooler conditions, which is manifested by a lowering of the forest line from 3.3 Ma on. The increase of Amaranthaceae pollen after 3.1 Ma suggests drier conditions along the coast. A comparison with mid‐Piacenzian warm period (mPWP) modeling shows that data and models agree regarding a drier coastal climate during the mPWP. The isochronous occurrence of environmental changes in the presented record, that is, cooling and coastal drying, with the first major pulse of ice‐rafted debris and cooling temperatures in the Northern Hemisphere (between 3.28 and 3.31 Ma) suggests that these changes might have been a precursor of the intensification of the Northern Hemisphere glaciation. Plain Language Summary Around 3.6 to 2.6 million years ago was a time in Earth's history when many conditions were similar to today like the arrangement of the continents, vegetation, and greenhouse gas concentrations. However, the Arctic was not yet covered by ice sheets, and the global climate was warmer, making this time period well suited to study the drivers and mechanisms of warm climates. Towards the end of this period, global climate started to cool, which is known mainly from Northern Hemisphere climate reconstructions. We wanted to find out if cooling and drying conditions found in other records also occurred in the South American tropics. Fossil pollen assemblages preserved in marine sediments can tell us which plants grew in our study area (broadly western Ecuador). Since plants hav