SEMINAR: MGG Student Seminar TODAY 12-1 PM, Amanda Oehlert & Wenliang Zhao

[Noelle Van Ee <NVanee@rsmas.miami.edu>]

MGG STUDENT SEMINAR

WEDNESDAY, February 21

12:00 - 1:00 PM

SLAB SEMINAR ROOM

Multiple Geochemical Records Suggest a Period of Ocean Anoxia during the Lower Mississippian 

Amanda M. Oehlert 

Ocean anoxic events represent major geological, biological, and chemical changes in the global oceans that contributed to a water column that was at least partially devoid of oxygen at certain times in Earth’s history.  Some periods of ocean anoxia are thought to be caused by enhanced productivity in the surface ocean, which prevents the rapid degradation of organic material in the water column. As a result, ocean anoxic events are often identified in outcrops and cores as the deposition of a widespread black layer. While these events are numerous in the Cretaceous record of global carbon cycling, a Paleozoic event has recently been investigated. Geochemical records from Lower Mississippian carbonate strata in North America and Europe suggest a period of global ocean anoxia, but the tell-tale black layer has not been encountered.

In this presentation, the possibility that an ocean anoxic event occurred during the Lower Mississippian will be evaluated. This hypothesis will be tested in ramp carbonates from the Lower Mississippian that outcrop in the Western United States. Previously published geochemical evidence from the Madison Limestone suggests that a period of ocean anoxia occurred at the Kinderhookian-Osagean stage boundary. This presentation will synthesize the results of three types of independent geochemical records including; the isotopic composition of inorganic carbonate (d¹³C_carb), organic carbon (d¹³C_org), and carbonate associated sulfate (d³⁴S_CAS). These three records have been reconstructed for two outcrops along the transect, a more distal ramp outcrop called Sheep Mountain, and an inner ramp location called Wind River Canyon.

Simultaneous enrichments of both of the carbon isotope records (d¹³C_carb and d¹³C_org) observed at the two locations support the interpretation of an ocean anoxic event. The results of the d³⁴S_CAS analysis are less clear. At the distal location, Sheep Mountain, all three records covary through time (d¹³C_carb, d¹³C_org, and d³⁴S_CAS), further supporting the interpretation that a period of anoxic conditions on the Madison Limestone ramp existed. However, the pattern of changes in the d³⁴S_CAS record did not correlate with the co-varying trends in the d¹³C_carb and d¹³C_org records at Wind River Canyon. In fact, the d³⁴S_CAS record appears to be inversely correlated with the d¹³C_carb and d¹³C_org records.

The lack of a similar trend in d³⁴S_CAS values at Sheep Mountain and Wind River Canyon may be the result of diagenetic alteration of the d³⁴S_CAS values. Based on these preliminary results, the ability of the d³⁴S_CAS values to faithfully record paleo-seawater conditions is questionable. However, the covariation of the d¹³C_carband d¹³C_org records suggests that a period of anoxia likely occurred during the deposition of the Madison Limestone in North America.  Further work on the Madison Limestone will be conducted to investigate this observation.

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Rapid Ice Loss at Vatnajokull, Iceland Detected by InSAR

Wenliang Zhao

Icelandic post-glacial rebound already finished at about 9000 BP. Due to rapidly present day ice melting, GIA signal near Vatnajokull -- the largest ice cap in Europe -- is very strong. Varies of geodetic methods have been used to study on GIA at Vatnajokull such as leveling, GPS, gravity. We present a new method for GIA study using satellite synthetic aperture radar interferometry. 15 year (1995-2009) ERS data was used for generation InSAR time series. We use both ascending & descending data to retrieve vertical displacement.

1) The method to retrieve vertical displacement is well known for several years. However, there is no theoretical model for vertical time series because usually we do not have data from both ascending and descending tracks at the same date. Here we introduce a theoretical model to generate InSAR vertical time series. We test the model using both simulated data and real data and compare with a simple method based on spline interpolation. The proposed method works fines. We monitored up to 15 mm/yr relative uplift at the edge of Vatnajokull. 

2)  We use a elastic half space model to estimate ice loss at Vatnajokull based on a coarse ice model (Sigmundsson et al., 1992). Discussion about relationship between Young's modulus & ice loss shows they are correlated to each other that is not easy to be separated. Based on seismic records and previous study, we estimate an average ice loss of 75 cm/yr from 1995-2009 by fixing Young's modulus. Compared with glaciological record based on ground observation, the model works pretty good.