1. MILANKOVITCH SCALE CYCLICITY DURING CRETACEOUS OCEANIC ANOXIC EVENT 2 AT DEMERARA RISE, WESTERN EQUATORIAL ATLANTIC OCEAN
Sean Karoly, Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, Fairfax, VA, 22030
GSA Northeast Section, Pittsburgh, PA
March 21, 2017

2. The Cretaceous Greenhouse
94 MYA – Mid-Cretaceous
O2 = 22.5% (today 21%)
Oceans susceptible to anoxia
CO2 = 1438 ppm (today ~ 400 ppm)
No ice on poles – high sea level
Atlantic Ocean opening
EarthViewer – Paleographic maps by C.R. Scotese, ©2012 PALEOMAP Project

3. Cretaceous Ocean Anoxic Events
C/T boundary
Images from other data? Such as CIE, Susceptibility, gamma ray – showing changes in data correlation from site to site. Mention more efficient means of correlation by using Milankovitch Cycles? Apparent duration of OAE2 on this image.
Temperature peaking at CT boundary, oxygen + carbon excursions, paleotemperature.
Apex of greenhouse at OAE2 and CT Boundary + increased carbon burial in oceans
Friedrich et al. 2012

4. Milankovitch Cycles as a global correlation tool
Astronomical forcing of climate is global, and should be recorded everywhere
Eccentricity: 100 kyr and 400 kyr cycles
Obliquity: ~41 kyr cycles
Precession signal strengthening/weakening?
Meyers, Isoastro Workshop 2016
Precession: ~21 kyr cycles

5. Demerara Rise ODP Leg 207, Sites 1257-1261 Site 1261:
Shipboard Scientific Party, 2004
Use prospectus etc. to state initial goals of drilling and why demerara rise is important.
Figure F3. Global paleogeographic reconstruction of the Late Cretaceous, showing the position of Demerara
Rise in the early South Atlantic.
Figure F1. Regional location map. The boxed area represents the approximate Leg 207 operational area on
Demerara Rise, which is shown in detail on Figure F2, p. 59.
Figure F2. Seismic survey track lines over bathymetry. The bathymetry was compiled from the seafloor pick
of the seismic data. Map modified after
ODP Leg 207, Sites
Site 1261:
Thickest OAE2 interval
Low incidence of coring gaps
Definitive nannofossil stratigraphy
Carbon isotope excursion (CIE)
Hardas and Mutterlose, 2006

6. Demerara Rise Site 1261 Lithology
ODP Log reports to describe black shales and what’s around them, thin sections? Economic importance
for Lithostratigraphy
Point out rock types according to ODP description?
Diagenetic calcite does recur and don’t contribute to Milankovitch signal

7. Black Shales of Demerara Rise
Age
Sites
66 Ma
85m of black shale
There are source rocks that have oil still in them (oil never left – Eagle Ford).
Source rock is any rock that will generate hydrocarbons (or has done so in the past).  Most source rock is gray or black shale. The material in the rock that will form oil is the organic material that is incorporated in the rock when it is formed.  The higher percentage of organic material in the rock (i.e. marine plankton) , the more oil it is capable of forming.
93.9 Ma
113 Ma
DARK
LIGHT
Hardas and Mutterlose 2006
10 cm section from Site B, section 2

8. Splicing grayscale data with Correlator
Site 1260 (Demerara Rise) A B
A+B splice
ImageJ to extract grayscale and RGB data
Correlator splices adjacent holes to remove gaps
Core A has gap at 432.5m depth
Core B complete at this interval, and is spliced into Core A.
Core A desired for its greater variability in color

9. Using Grayscale to detect Milankovitch Cycles
OAE 2
Strong meter scale cycles
etc.
Site B Section 3 XRF scan
Explain what grayscale is, and how it can be used as a proxy in correlation with other data?
Label XRF section for following slide
Eventually confirm location of 13B Section 3

10. Grayscale vs. biostratigraphy & carbon isotopes
C-T Boundary
(Meyers 2012)
A, B, C, D Correlation points to other sites (Erbacher 2005) D
Q. gartneri FO
mcd
Q. gartneri FO
mbsf
Gartneri doesn’t always occur in the same place as CIE – either CIE is not a global signature, or gartneri is not always preserved consistently. Need a tiebreaker – Milankovitch Cycles (global signature) so they should be everywhere to determine where boundary is. Have to do Milankovitch at multiple sites – if we can find cycles at all of them, correlate cycles to see if carbon isotopes are at the same time (probably more reliable than fossil occurrences).
A. albianus LO mcd C
A. albianus LO mbsf B A
FO and LO data from Hardas & Mutterlose, 2006

11. Comparison to Pueblo, CO (Western Interior Seaway)
GSSP
close to D
close to C
Discrepancy in boundary age with respect to CIE and point D. Gartneri – is it reliable? Tiebreaker = Milankovitch Cycles. Resolve differences in chemostratigraphy via Milankovitch Cycles
Hardas and Mutterlose 2006, adapted from Tsikos et al. 2004

12. Grayscale Correlation with XRF Data
Elements associated with
terrigenous influx
XRF Scan of 1.25 m section of 1261B, Core 13 Section 3 Si Fe
XRF scanning at 100 micron steps
Represents approx. 90 kyr (sed rate: 1.53 cm/kyr; see next slide)
Will be presented next by Baddouh and Hinnov K
Counts (to be converted to weight %) Ti Al
Section 3: Y-axis in counts – work in progress to convert to weight %.
Fe possibly recycled pyrite from ocean or weathering.
This section given 125 cm = 90 kyr.
XRF determines the chemical composition of a sample by measuring the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source.

13. Power Spectrum (MATLAB)
Average spectral misfit (R)
Preliminary analysis of 40m excerpt from ODP
FMS logging data
Long eccentricity (429 kyr)
Grayscale scan of m section of Site 1261
OAE2: 9.5m in 1261 = ~620 kyr
6.55m
Power Spectrum (MATLAB)
Eventually do ASM for my own data.
Application of ASM sedimentation rate converts to corresponding periodicities.
ASM method seeks to identify the sedimentation rate that minimizes the misfit between the
observed frequencies and predicted orbital components.
The statistical average of a certain signal or sort of signal (including noise) as analyzed in terms of its frequency content, is called its spectrum.
Short eccentricity (90.9 kyr)
1.39m
Spectral Density
Obliquity (42.9 kyr)
0.655m
Precession (19.6 kyr)
0.299m
Frequency (cycles/m)

14. Demerara Rise Site 1261 vs. Laskar theoretical eccentricity
429 kyr
2p MTM power spectrum for grayscale scan, 560 to 660 mbsf
Demerara Rise Site 1261 vs. Laskar theoretical eccentricity
91 kyr
43 kyr
20 kyr
Frequency (cycles/m)
OAE2
Depth (mbsf)
Box/outline correlations
A spectrogram is a visual representation of the spectrum of frequencies in a sound or other signal as they vary with time or some other variable. 
Evolutive harmonic analysis (EHA) is a time–frequency technique that can identify temporal changes in the dominant frequencies preserved in cyclic stratigraphic sections.
EHA may be utilized to assess sedimentation rate changes whenever periodic forcing phenomena can be readily identified (i.e., orbital forcing, annual varving, etc.).
Back in time
Downcore
Spectogram of Demerara Rise Site 1261 grayscale scan
Spectrogram of theoretical orbital eccentricity, Laskar 2004

15. Timescale discrepancy of OAE2
1261B
Radiochronology vs. biostratigraphy vs. isotopes etc.; CT boundary changes overtime and potential future changes. Li et al Change in Q. gartneri from site to site? Sedimentation rate a means of uncertainty?
OAE2
Erbacher et al. 2005:
kyr
Meyers et al. 2012:
kyr
Li et al. 2017:
820 +/- 25 kyr
Karoly (this study):
620 kyr

16. Further work
Confirm/deny Q. gartneri FO as a reliable global biomarker for the Cenomanian-Turonian boundary
Match XRF to photoscans for Site 1261
Complete Site 1260 analyses?
Define precise timescale for OAE2 at Demerara Rise
Improve understanding of paleo-oceanographic processes during the Mid-Cretaceous
Role of Atlantic Ocean gateway in producing Cretaceous black shales
Identify heat transport and ocean circulation
Cite work on 1260, any unanswered questions, better understanding of oceanographic processes

17. Eldrett 2015