Northern Hemisphere Glaciation during the Globally Warm Early Late Pliocene

The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Here we propose a conceptual model for the glaciation and deglaciation of MIS M2 based on geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect. Our records show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the continental ice sheets during MIS M2, despite near-modern atmospheric CO2 concentrations. Sea level drop during this glaciation halted the inflow of Pacific water to the Atlantic via the Central American Seaway, allowing the build-up of a Caribbean Warm Pool. Once this warm pool was large enough, the Gulf Stream–North Atlantic Current system was reinvigorated, leading to significant northward heat transport that terminated the glaciation. Before and after MIS M2, heat transport via the North Atlantic Current was crucial in maintaining warm climates comparable to those predicted for the end of this century

Distribution and carbon isotopic composition of diploptene from epiphytic bryophytes in Wuhan, central China

Diploptene is a ubiquitous hopanoid in the geosphere, synthesized by all hopanoid-containing bacteria. Variations in the concentration and stable carbon isotopic composition (δ13C) of diploptene in ancient peats and lignite can be used to reconstruct certain aspects of the wetland methane cycle in the past. However, the sources and mechanisms that control diploptene δ13C values in wetlands are not fully constrained. To address this, here we determined the distribution and δ13C values of diploptene, as well as n-alkanes, obtained from five genera of epiphytic bryophytes (non-vascular plants such as mosses) that occupy-three different habitats: soil, rock, and tree bark. Our data show that the concentrations of diploptene are highly variable with two order of magnitude differences between the various species. Mosses collected from the soil habitat had higher concentrations compared to those from rock and tree habitats. This suggests that the input from some habitats might dominate the sedimentary signal. The δ13C values of diploptene (δ13Cdip) also vary between species with values ranging between –39.2‰ and –31.2‰. Generally, the δ13C values of diploptene and long chain n-alkanes (i.e., C29 and C31) are similar (±2‰) in most of the bryophyte species. This may suggest that diploptene is produced by heterotrophic bacteria that live in symbiosis with the mosses. However, for some bryophytes the δ13Cdip values are much more 13C depleted (>–2‰) compared to long chain n-alkanes, implying that for some mosses bacterial methanotrophs or methylotrophs may contribute to the diploptene pool. Our findings expand our understanding of the biological sources of diploptene in terrestrial epiphytic bryophytes, which will allow for a more detailed interpretation of the long chain n-alkanes and diploptene (δ13C values) in past environmental and paleoclimatic reconstructions

Warming drove the Expansion of Marine Anoxia in the Equatorial Atlantic during the Cenomanian Leading up to Oceanic Anoxic Event 2

Oceanic Anoxic Event (OAE) 2 (~93.5 millions of years ago) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water column anoxia, water column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 million years of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) contents are high but variable (0.41&ndash;17 wt. %) across the Cenomanian and increase with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ~ 35 to 40 &deg;C as well as the episodic occurrence of 28,30-dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene, but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming.</p

Depth related variation of isoprenoid and hydroxylated tetraether lipids in Lake Lugu, Southwest China:Implications for palaeoenvironmental reconstructions

Archaeal glycerol dibiphytanyl glycerol tetraethers (isoGDGTs) and their hydroxylated derivatives (OH-GDGTs) have been increasingly applied to reconstruct past changes in lake temperature and lake-level using down-core sediments. However, a detailed examination of the distribution pattern of iso- and OH-GDGTs in lacustrine sediments is so far limited. To investigate the controls on the sedimentary GDGT distribution in lakes, we examined the archaeal GDGT distribution in surface sediments at different water depths from Lake Lugu, a deep alpine lake in southwest China. Our aim is to determine their distribution, sources and controlling factors. Based on the significant correlations between iso- and OH-GDGTs in deep-water sediments (&gt; 20 m), we suggest that the main biological source of archaeal GDGTs in surface sediments is aquatic Group I.1a Thaumarchaeota (Nitrosoarchaeum). The depth-related variation of iso- and OH-GDGTs indicates that water depth is the main factor affecting the distribution of archaeal GDGTs in Lake Lugu, reflecting that Thaumarchaeota prefer to live in the deeper layer above the oxycline. This relationship leads to a positive correlation between %Cren, %OH-GDGTs, and Cren/Cren’ with water depth, confirming their potential application for paleo-lake level reconstruction. Our study improves the understanding of the factors that control the archaeal GDGTs in a deep alpine lake and suggest that they might be used as lake-level indicators