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The tight coupling between temperature and atmospheric CO2 is shown by ice core records for the past 0.8 million years (Myr). However, the modern atmospheric partial pressure of CO2 (pCO2) has exceeded previous interglacial pCO2 levels over the past 0.8 Myr, suggesting that the earlier part of the Pleistocene and Pliocene might be a better analog of today's radiative forcing of CO2. The early Pleistocene experienced a constant cooling characterized by the intensification of northern hemisphere glaciations. Existing pCO2 records developed from marine organic matter and inorganic precipitates, however, disagree with the trends and absolute values of CO2 over this time interval. Here we present quantitative interglacial pCO2 estimates from ∼ 2.6 – 0.9 Myr using the stable carbon isotopic compositions of pedogenic carbonates collected from the Chinese Loess Plateau (CLP). Our pCO2 records provide the first documentation of pCO2 from continental sedimentary deposits over the early Pleistocene. The successive decrease of our pCO2 records is broadly consistent with the increase in deep-sea δ O 18 and the overall decline of sea surface temperature (SST) at this time, but in contrast with the increasing peak interglacial pCO2 recorded in ice cores for the last 0.8 Myr.

TEX86 [TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms] has been widely applied to reconstruct (paleo-) sea surface temperature. Marine Group I (MG-I) Thaumarchaeota were thought to be the primary source of GDGTs constituting the TEX86 formula; however, recent research has suggested that Marine Group II (MG-II) Euryarchaeota may also contribute significantly to the GDGT pool in the ocean. Little is known regarding the potential impact of MG-II Euryarchaeota-derived GDGTs on TEX86 values recorded in marine sediments. In this study, we assessed the relationship between distributions of GDGTs and MG-II Euryarchaeota and evaluated its potential effect on the TEX86 proxy. Lipid and DNA analyses were performed on suspended particulate matter and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (SCS, seawater). TEX86-derived temperatures from the water column and surface sediments were significantly correlated and both were lower than satellite-based temperatures. The ring index (RI) values in these environments were higher than predicted from the calculated TEX86-RI correlation, indicating that the GDGT pool in the water column of the PR estuary and coastal SCS comprises relatively more cyclopentane rings, which thereby altered TEX86 values. Furthermore, the abundance of MG-II Euryarchaeota 16S rRNA gene in the mixing water was two to three orders of magnitude higher than those observed in the river or seawater. Significant linear correlations were observed between the gene abundance ratio of MG-II Euryarchaeota to total archaea and the fractional abundance of GDGTs with cyclopentane rings. Collectively, these results suggest that MG-II Euryarchaeota likely produce a large proportion of GDGTs with 1-4 cyclopentane moieties, which may bias TEX86 values in the water column and sediments. As such, valid interpretation of TEX86 values in the sediment record, particularly in coastal oceans, should consider the contribution from MG-II Euryarchaeota.

Old Gradients The surface ocean temperature gradient between the warmer Western Equatorial Pacific and the cooler Eastern Equatorial Pacific is smaller during El Niño episodes than during neutral periods or during La Niñas. Some reconstructions of Pacific Ocean sea surface temperatures (SST) covering periods before ∼3 million years ago have suggested a permanent El Niño–like state. Zhang et al. (p. 84 ; see the Perspective by Lea ) present data from a biomarker-derived proxy for SST that indicate a sizable east-west gradient has existed for the past 12 million years, contradicting the concept of a permanent El Niño–like state existed.

Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility +/- 3-4 degrees C when translated to temperature) but a large spread in BIT measurements (reproducibility +/- 0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0 degrees C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the true (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

Gas hydrates represent one of the largest pools of readily exchangeable carbon on Earth's surface. Releases of the greenhouse gas methane from hydrates are proposed to be responsible for climate change at numerous events in geological history. Many of these inferred events, however, were based on carbonate carbon isotopes which are susceptible to diagenetic alterations. Here we propose a molecular fossil proxy, i.e., the “Methane Index (MI)”, to detect and document the destabilization and dissociation of marine gas hydrates. MI consists of the relative distribution of glycerol dibiphytanyl glycerol tetraethers (GDGTs), the core membrane lipids of archaea. The rational behind MI is that in hydrate-impacted environments, the pool of archaeal tetraether lipids is dominated by GDGT-1, -2 and -3 due to the large contribution of signals from the methanotrophic archaeal community. Our study in the Gulf of Mexico cold-seep sediments demonstrates a correlation between MI and the compound-specific carbon isotope of GDGTs, which is strong evidence supporting the MI-methane consumption relationship. Preliminary applications of MI in a number of hydrate-impacted and/or methane-rich environments show diagnostic MI values, corroborating the idea that MI may serve as a robust indicator for hydrate dissociation that is useful for studies of global carbon cycling and paleoclimate change.

The late Pliocene onset of major Northern Hemisphere glaciation (NHG) is one of the most important steps in the Cenozoic global cooling. Although most attempts have been focused on high-latitude climate feedbacks, no consensus has been reached in explaining the forcing mechanism of this dramatic climate change. Here we present a key low-latitude climate record, the high-resolution Asian monsoon precipitation variability for the past fi ve million years, reconstructed from South China Sea sediments. Our results, with supporting evidence from other records, indicate signifi cant mid-Pliocene Asian monsoon intensifi cation, preceding the initiation of NHG at ca. 2.7 Ma ago. This 1.4-million-year-long monsoon intensifi cation probably enhanced monsoon-induced Asian continental erosion and chemical weathering and in the process left fi ngerprints in marine calcium isotopes. Furthermore, increased rock weathering and/or organic carbon burial probably lowered the contemporary atmospheric CO 2 and may have triggered the NHG onset.

Precipitation has a larger variability than temperature in tropical monsoon regions, thus it is an important climate variable. However, reconstructions of long-term rainfall histories are scarce because of the lack of reliable proxies. Here we document that iron oxide minerals, specifically the ratio of hematite to goethite (Hm/Gt), is a reasonable precipitation proxy. Using diffuse reflectance spectrophotometry, we measured samples from Ocean Drilling Program (ODP) 1143 drilling site (9°21.72′N, 113°17.11′E, 2777 m water depth) for hematite and goethite, whose formation processes are favored by opposing climate conditions. In order to determine the content of hematite and goethite we produced a set of calibration samples by removing the iron oxides to generate the natural matrix to which hematite and goethite in known percentages were added. From these calibration samples we developed a transfer function for determining hematite and goethite concentration from a sample's spectral reflectance. Applying this method to ODP 1143 sediments (top 34 m of a 510 m core with sampling interval of 10 cm) we were able to reconstruct a continuous precipitation history for SE Asia of the past 600 kyr using the Hm/Gt ratio as a proxy of the precipitation variability of Asian monsoon. The reliability of this Hm/Gt proxy is corroborated by its consistency with the stalagmite δ18O data from South China. Comparing long-term Hm/Gt records with the surface temperature gradient of equatorial Pacific Ocean, we found that monsoon precipitation and El Nino are correlated for the last 600 kyr. The development of El Nino-like conditions decreased SE Asia precipitation, whereas precipitation increases in response to La Nina intensification.

Cyclization in glycerol dibiphytanyl glycerol tetraethers (GDGTs) results in internal cyclopentane moieties which are believed to confer thermal stability to crenarchaeal membranes. While the average number of rings per GDGT lipid (ring index) is positively correlated with temperature in many temperate environments, poor correlations are often observed in geothermal environments, suggesting that additional parameters may influence GDGT core lipid composition in these systems. However, the physical and chemical parameters likely to influence GDGT cyclization which are often difficult to decouple in geothermal systems, making it challenging to assess their influence on lipid composition. In the present study, the influence of temperature (range 65–81°C), pH (range 3.0–5.0), and ionic strength (range 10.1–55.7 mM) on GDGT core lipid composition was examined in the hyperthermoacidophile Acidilobus sulfurireducens, a crenarchaeon originally isolated from a geothermal spring in Yellowstone National Park, Wyoming. When cultivated under defined laboratory conditions, the composition of individual and total GDGTs varied significantly with temperature and to a lesser extent with the pH of the growth medium. Ionic strength over the range of values tested did not influence GDGT composition. The GDGT core lipid ring index was positively correlated with temperature and negatively correlated with pH, suggesting that A. sulfurireducens responds to increasing temperature and acidity by increasing the number of cyclopentyl rings in GDGT core membrane lipids.

Contrary to our conclusions, Ravelo et al . argue that our TEX 86 -based sea surface temperature (SST) records do not conflict with the supposition of “permanent El Niño–like” conditions during the early Pliocene. We show that the way Ravelo et al . treat the existing temperature data perpetuates an inaccurate impression of cooler Pacific warm-pool SSTs and low equatorial temperature gradients in the past.

The alkenone– p CO 2 methodology has been used to reconstruct the partial pressure of ancient atmospheric carbon dioxide ( p CO 2 ) for the past 45 million years of Earth's history (Middle Eocene to Pleistocene epochs). The present long-term CO 2 record is a composite of data from multiple ocean localities that express a wide range of oceanographic and algal growth conditions that potentially bias CO 2 results. In this study, we present a p CO 2 record spanning the past 40 million years from a single marine locality, Ocean Drilling Program Site 925 located in the western equatorial Atlantic Ocean. The trends and absolute values of our new CO 2 record site are broadly consistent with previously published multi-site alkenone–CO 2 results. However, new p CO 2 estimates for the Middle Miocene are notably higher than published records, with average p CO 2 concentrations in the range of 400–500 ppm. Our results are generally consistent with recent p CO 2 estimates based on boron isotope-pH data and stomatal index records, and suggest that CO 2 levels were highest during a period of global warmth associated with the Middle Miocene Climatic Optimum (17–14 million years ago, Ma), followed by a decline in CO 2 during the Middle Miocene Climate Transition (approx. 14 Ma). Several relationships remain contrary to expectations. For example, benthic foraminiferal δ 18 O records suggest a period of deglaciation and/or high-latitude warming during the latest Oligocene (27–23 Ma) that, based on our results, occurred concurrently with a long-term decrease in CO 2 levels. Additionally, a large positive δ 18 O excursion near the Oligocene–Miocene boundary (the Mi-1 event, approx. 23 Ma), assumed to represent a period of glacial advance and retreat on Antarctica, is difficult to explain by our CO 2 record alone given what is known of Antarctic ice sheet history and the strong hysteresis of the East Antarctic Ice Sheet once it has grown to continental dimensions. We also demonstrate that in the Neogene with low CO 2 levels, algal carbon concentrating mechanisms and spontaneous biocarbonate–CO 2 conversions are likely to play a more important role in algal carbon fixation, which provides a potential bias to the alkenone– p CO 2 method.