- Chapter contents
- Abstract
- Introduction
- Methods and materials
- Analysis
-
Results and discussion - Summary
- Acknowledgments
- References
-
Figures
-
Tables
- PDF file
doi:10.2204/iodp.proc.342.205.2017
Results and discussion
Continental input of GDGTs
At Site U1406 in the lower part of the studied succession (Chrons C11r–C9n), the BIT index shows a gradually decreasing trend from ~0.55 to ~0.2, spanned by two local maxima within Chrons C11n.2n (0.8 ≤ BIT ≤ 0.88) and C9n (0.44 ≤ BIT ≤ 0.65). Within Chrons C8r and C8n.2n, the BIT index shows moderate values scattered between 0.25 and 0.65. Across the Oligocene–Miocene transition, the BIT index shows a sharp decrease from 0.6 to 0.1 (Fig. F3).
The value range for the BIT index at Site U1411 lies between 0.3 and 0.6 and shows elevated values (0.55–0.65) in the interval from the top of Chron C13n to the middle part of Chron C12r (Fig. F4).
The overall moderate to high BIT values indicate that in the GDGT pool a substantial part of the GDGTs are terrestrially derived because open marine sediment typically has BIT values < 0.1 (Schouten et al., 2013, and references cited therein). In general, we observe that BIT index values are higher in the uppermost Rupelian than in the lower Chattian (Figs. F3, F4) and thus may imply a decrease in the input of soil organic matter to the site around the Rupelian/Chattian boundary.
Impact of methane cycling
At both sites, the MI is between 0.2 and 0.27. The values for both studied successions are fairly similar to the average values for Eocene–Oligocene deposits shown by Inglis et al. (2015). This suggests that the Oligocene strata at the Newfoundland sediment drift are, like the majority of the age-equivalent deposits, relatively unaffected by the input of GDGTs from Euryarchaeota, which are associated with the anaerobic oxidation of methane (Pancost et al., 2001; Blumenberg et al., 2004; Inglis et al., 2015).
At Site U1406, the %GDGT-0 index shows values between 30% and 49% (Fig. F3), with no apparent trend in time. One local maximum within Chron C11n.2n with index values of up to 71.5% (Sample 342-U1406A-19H-2, 76–78 cm) was observed. Furthermore, two isolated samples show values >52% (147.84 meters composite depth [mcd], Sample 342-U1406B-15H-4, 76–78 cm; and 167.09 mcd, Sample 17H-3, 76–78 cm). At Site U1411, the %GDGT-0 index values range from 37% to 45%. We observed a gradual, gently increasing trend from 37% in Chron C13r (the base of the studied succession at Site U1411) to 44% in the lower part of Chron C12r. From the middle part of Chron C12r to Chron C8n.2n (the top of the studied succession at Site U1411), the values are rather stable, between 40% and 45%. One sample (54.62 mcd, Sample 342-U1411B-7H-4, 26–30 cm) shows a %GDGT-0 index value of 50%.
Our results indicate that only one sediment sample has a %GDGT-0 > 67% (Sample 342-U1406A-19H-2, 76–78 cm), which suggests that some of the GDGT may be derived from methanogenic archaea in that particular sample. In contrast, all other sediment shows no sign of input of methanogenic archaea. Thus, the combined MI and %GDGT-0 index indicate that there was no significant impact of archaea involved in the methane cycle on the distribution of GDGTs and that most of the GDGTs are likely to derive from Thaumarchaeota, especially since crenarchaeol, the biomarker for Thaumarchaeota, is present in relatively high abundances (40%–50%; Tables T1, T2). Only in five samples from Site U1406 were abundances <40%, but these samples also had high BIT values (0.6–0.8). This agrees with the general observation that the %crenarchaeol (of total isoGDGTs) is much lower in soil organic matter than marine organic matter (see compilation in Schouten et al., 2013).