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LATE MIDDLE ORDOVICIAN KUKERSITES: NEGATIVE d15N EXCURSION COINCIDES WITH POSITIVE d13C EXCURSION
A sequence of Iowa kukersites formerly described in [1] was analyzed for d15N and IR spectroscopy of kerogens. d15N generally decreases up-core from 7.1 to 4.6 , whereas d13C of kerogen increases over the same interval from -31.6 to -22.7 [1]. The positive carbon excursion was attributed to enhanced productivity and better preservation of organic matter (OM) at the end of Middle Ordovician. The opposite nitrogen isotopic excursion corroborates the earlier view and suggests one or more of the following paleoceanographic circumstances surrounding enhanced productivity:
Ø Exhaustion of available nitrate led to biological fixation of elemental N2 (d15N » 0 ) with little isotope fractionation.
Ø Oxygen depletion in the water column caused denitrification with (strong) isotope fractionation, resulting in residual 15N-enriched NO3- available for biological uptake.
Kerogens were also analyzed by IR spectroscopy to quantify aromaticity (Ar) and the CH2/CH3 ratio (reflecting the length and branching of aliphatic chains). These two IR-parameters show an inverse relationship with depth; Ar has the maximum at intermediate depth, and CH2/CH3 reveals that deepest and shallowest kerogens contain a more pronounced straight-chain aliphatic moiety. Lithology of sediments changes from shale at the bottom of core through limestone to shale and shaly-limestone at the top [1]. Changes in IR-parameters seem to be related to lithology; shale and shaly-limestone correspond to OM characterized by low aromaticity and long aliphatic chains, and limestone corresponds to more aromatic OM with shorter aliphatic chains. Correspondence between IR-parameters and lithology suggests that the investigated OM was produced locally, therefore the observed changes reflect local changes in paleoenvironment. The fact that isotopic and IR-trends are not parallel suggests that d-values were influenced by factors on a geographically wider scale.
| Depth [m] | d13C [] | d15N [] | Ar | CH2/CH3 |
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|
|
|
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| 289.6 | -22.7 | 4.8 | nd | nd |
| 289.9 | -23.1 | nd | 0.00042 | 6.1 |
| 290.0 | -23.4 | 4.6 | nd | nd |
| 290.4 | -22.9 | 5.3 | 0.00048 | 5.6 |
| 290.8 | -24.8 | nd | 0.00053 | 5.9 |
| 291.5 | -27.1 | 4.6 | 0.00094 | 2.8 |
| 295.5 | -30.3 | nd | 0.00248 | 3.2 |
| 304.4 | -31.0 | 5.8 | 0.00141 | 3.5 |
| 307.2 | -31.7 | 5.4 | nd | nd |
| 308.0 | -31.5 | nd | nd | nd |
| 308.5 | -31.1 | nd | 0.00057 | 4.4 |
| 311.2 | -31.6 | 7.1 | nd | nd |
nd=not determined
Reference:
[1] Hatch et al., 1987. AAPG Bull. 71, 1342-1354.