Stable warm tropical climate through the Eocene Epoch

Abstract

Earth's climate cooled from a period of extreme warmth in the early Eocene Epoch (ca. 50 Ma) to the early Oligocene (ca. 33 Ma), when a large ice cap fi rst appeared on Antarctica. Evidence from the planktonic foraminifer oxygen isotope record in deep-sea cores has suggested that tropical sea-surface temperatures declined by 5-10 degrees over this interval, eventually becoming much cooler than modern temperatures. Here we present paleotemperature estimates from foraminifer isotopes and the membrane lipids of marine Crenarcheota from new drill cores in Tanzania that indicate a warm and generally stable tropical climate over this period. We reinterpret the previously published isotope records in the light of comparative textural analysis of the deep-sea foraminifer shells, which shows that in contrast to the Tanzanian material, they have been diagenetically recrystallized. We suggest that increasingly severe alteration of the deep-sea plankton shells through the Eocene produced a diagenetic overprint on their oxygen isotope ratios that imparts the false appearance of a tropical sea-surface cooling trend. This implies that the long-term Eocene climatic cooling trend occurred mainly at the poles and had little effect at lower latitudes.

Modeling the influence of a reduced equator-to-pole sea surface temperature gradient on the distribution of water isotopes in the Early/Middle Eocene

“…Tropical Eocene sea surface temperatures simulated by the fully coupled CCSM1.4 Eocene run of Huber and Nof (2006) were found to be consistent with SST estimates of 33°C (Pearson et al, 2007) from Tanzanian sediments from the Early/Middle Eocene. These mean annual SSTs at 19ºS paleolatitude, were reconstructed based on TEX 86 measurements using the Schouten et al (2003) Fig.…”

“…During a 1.2 Myr period 49 Ma ago, the free floating aquatic fern Azolla grew and reproduced in the Eocene Arctic Speelman et al, 2009a), indicating that the surface waters of the Arctic Basin freshened considerably. During this so-called Azolla interval, tropical sea surface temperatures (SSTs) were somewhat warmer than today (with a mean annual temperature (MAT) of 32-34°C at 19°C) (Pearson et al, 2007), while Arctic SSTs were substantially higher (with a MAT of 10°C) . The consequently reduced temperature gradient between the equator and the poles and the presence of freshwater at the North Pole provide important boundary conditions for understanding the hydrological cycle and latent heat transport during this interval.…”

“…While earlier in the Eocene (from 55 to 50 Ma) temperature gradients may have been even lower (e.g. Pearson et al, 2007;Sluijs et al, 2008), data coverage in the Azolla interval centered near 49 Ma is more complete and we thus focus on that interval.…”

Climate modelling and deep-time climate change

“…High-latitude continental interiors were also much warmer than today (Greenwood and Wing, 1995 ). In the tropics, recent sea-surface temperature reconstructions using δ 18 O in well-preserved foraminifera indicate temperatures in excess of 30 °C, independently confi rmed by TEX86 estimates (Pearson et al, 2007 ). Overall, these reconstructions suggest a global mean temperature during the EECO of perhaps 25 °C, around 10 °C higher than today.…”

“…While older temperature reconstructions from pelagic foraminifera (e.g. Zachos et al, 1994 ) showed Eocene tropical temperatures comparable to or even lower than today, more recent studies using exceptionally well-preserved forams indicate much higher temperatures (Pearson et al, 2007 ), suggesting that the colder estimates are biased low due to diagenesis. With these new tropical temperature estimates, the low-gradient paradox becomes much less severe.…”