- Chapter contents
- Abstract
- Introduction
- Methods and materials
- Analysis
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Results and discussion - Summary
- Acknowledgments
- References
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Figures
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Tables
- PDF file
doi:10.2204/iodp.proc.342.205.2017
Introduction
Glycerol dialkyl glycerol tetraethers (GDGTs) are lipids spanning the cell membrane of archaea and certain bacteria. GDGTs are characterized by a large structural diversity and are observed in a wide range of environments (e.g., soil, peat, marine water column, and hot springs) (Schouten et al., 2013, and references cited therein). Structurally, GDGT membrane lipids are divided into two main groups: isoprenoid and branched GDGTs (Fig. F1).
Isoprenoid GDGTs (isoGDGTs) contain an isoprenoid carbon skeleton and derive from archaea (e.g., Schouten et al., 2000). One of the most ubiquitous isoGDGTs is crenarchaeol, a unique membrane lipid with a cyclohexane moiety. Crenarchaeol is produced by the marine archaea belonging to the phylum Thaumarchaeota (Sinninghe Damsté et al., 2002; Pitcher et al., 2011). Most species of Thaumarchaeota have been shown to be chemoautotrophs and are ammonia oxidizers (e.g., Könneke et al., 2005; Wuchter et al., 2006). The Thaumarchaeota also synthesize other common and environmentally dependent isoGDGTs: GDGT-0 (with no cyclopentane moiety) and GDGT-x (x denotes the number of cyclopentane moieties: 1, 2 or 3; Fig. F1). GDGT-0 is, however, also synthesized by many other archaea including methanogens (i.e., microorganisms that produce methane as a metabolic byproduct in anoxic conditions), which sometimes are important contributors to the sedimentary GDGT pool (e.g., Pancost et al., 2001; Blumenberg et al., 2004). It is also hypothesized that some smaller amounts of GDGT-1, GDGT-2, and GDGT-3 in sediment can be derived from the sedimentary archaeal methanogens or methanotrophic Euryarchaeota (Schouten et al., 2013, and references cited therein).
Both isoGDGTs and brGDGTs are ubiquitous and rather well preserved in the sedimentary archives. Their omnipresence and good preservation in the geological record (for more details concerning the preservation, distribution, and origin of GDGTs, e.g., Schouten et al., 2004, 2007; Taylor et al., 2013; Inglis et al., 2015; Qin et al., 2015) are responsible for successful application of the GDGT-based indexes (e.g., TEX86 and MBT/CBT paleotemperature proxies; for most recent TEX86 reviews see Pearson and Ingalls, 2013; Tierney and Tingley, 2015) in the paleoclimatic and paleoenvironmental studies of the Paleogene (Schouten et al., 2008; Bijl et al., 2009; Donders et al., 2009). However, the first and crucial step is to recognize the source and relative abundance of the studied GDGTs. To this end, several indexes have been developed. The branched isoprenoid tetraether (BIT) index is expressed as the ratio between brGDGTs and the crenarchaeol. The index is used as a proxy for estimating the relative input of soil and river organic material into marine settings (Hopmans et al., 2004; Huguet et al., 2009; Zell et al., 2014). BIT values span from close to 0 (absence of brGDGTs, typical for open marine environments) to 1 (absence of crenarchaeol, characteristic for mineral soils and peat) (Hopmans et al., 2004; Schouten et al., 2013). This proxy has successfully been applied in numerous paleoenvironmental studies (e.g., Donders et al., 2009; Śliwińska et al., 2014). The methane index (MI; the relative ratio of GDGTs 1–3 versus crenarchaeol; Zhang et al., 2011) and %GDGT-0 (the ratio between GDGT-0 and crenarchaeol; Sinninghe Damsté et al., 2012) are used as indicators for the contribution of methanogenic and methanotrophic archaea. In “normal marine” environments MI ≤ 0.3, whereas in methane-rich environments MI can be as high as 1 (Zhang et al., 2011). Sinninghe Damsté et al. (2012) showed that if %GDGT-0 > 67%, then GDGTs will most probably have a methanogenic source. These indicators have been successfully applied to constrain the sources and distribution of GDGTs in upper Paleogene sediment (e.g., Inglis et al., 2015).
Here, we evaluated the distribution and sources of GDGTs within sediment cores retrieved during Expedition 342 at the Newfoundland sediment drift. Operations during Expedition 342 succeeded in obtaining high-quality spliced records of upper Eocene to lower Miocene strata (see the “Site U1406” and “Site U1411” chapters [Norris et al., 2014a, 2014b]). The sites are located in the area where the modern Deep Western Boundary Current and the Gulf Stream Current transect (for details see Norris et al., 2011).
The purpose of our study was to analyze the depositional paleoenvironment of the Oligocene deposits from the Newfoundland sediment drift as well as to identify the source and the distribution of isoGDGTs and brGDGTs.