1. Middle Island Sinkhole: Analysis of Sediment Cores
Clarissa Crist, Interdisciplinary Environmental Science & Society (IESS)
Faculty Sponsors: Dr. Christine Clark, Dr. Todd Grote, in association with Dr. Nathan
Sheldon from the University of Michigan—Ann Arbor
Abstract
Middle Island Sinkhole (MIS), located in Lake Huron, features dynamic
microbial mats that help to visualize how early microbes in shallow seas
could have oxygenated our planet 2.5 billion years ago. The unique
environment of the sinkhole is attributed to the sulfate-rich and oxygen-
poor groundwater that features a lower pH, lower temperature, and
higher conductivity than surrounding Lake Huron water. I participated in
processing sediment cores from MIS in order to analyze them for carbon
and nitrogen content, and different iron phases. This was done in order to
understand the redox chemistry of the sinkhole, and how it allows for the
development of the microbial mats. I also processed sediment samples
from El Cajon, a shallower onshore sinkhole that features similar
microbial activity, to understand what controls the growth of these
microbial mats.
Introduction
The reason for abundant microbes is sulfate-rich and oxygen-poor
groundwater, which vents into Lake Huron. This creates an ideal
environment for the growth of microbial mats with a wide variety of
different metabolisms, similar to the metabolisms of microbes 2.5
billion years ago.
MIS groundwater is denser and cooler than surrounding Lake Huron
water, with a lower pH and higher conductivity.
Objective: Comparing El Cajon and MIS to see of they have similar
carbon and nitrogen burial.
Hypothesis: El Cajon sediments will have similar carbon and nitrogen
contents as MIS sediments, and both will differ from carbon and
nitrogen contents of Lake Huron sediments.
Methods
Sample Collection: Sediment cores from MIS and a near-shore Lake Huron
site were collected by NOAA divers from May 2015—October 2015, and El
Cajon surficial sediments were collected by hand.
Sample Prep: MIS and Lake Huron sediment cores were sawed into 1-3 cm
increments. Those samples, and the El Cajon samples, were freeze-dried and
homogenized before being acid washed with HCl to remove any inorganic
carbon.
Sample Analysis: Samples were loaded into small tin capsules, and
combusted in a Costech Elemental Analyzer to determine carbon and nitro-
gen contents.
References
Biddanda, D. A. et al., (2012). Natural Education Knowledge 3(3):5
Ruberg, S., et al., (Winter 2008/2009). Marine Technology Society Journal, 42
(4), 12-21.
Acknowledgements
Dr. Nathan Sheldon for taking me in as an undergraduate research student under
his advisement.
Katy Rico, the graduate student I worked with, who guided me throughout this
process and taught me a great deal about this project.
Dr. Todd Grote and Dr. Christine Clark, who also advised me throughout this re-
search project and provided their insight into presenting my work.
Results and Conclusions
Figure 3 shows there is no significant statistical difference between the
C:N ratios of Lake Huron and El Cajon sediment samples.
However, figure 4 shows that some samples of El Cajon have low %Corg
and %N, like other Lake Huron results, and other El Cajon samples have
high %Corg and %N, like MIS results.
According to my results, my hypothesis is not correct and El Cajon is not
distinctively similar to either Middle Island Sinkhole or Lake Huron.
Future work would entail taking sediment core samples rather than
surficial sediment samples at El Cajon in order to conduct a more
thorough investigation of nutrient cycling.
After nutrient work, we would look into sulfur and iron content of the
sediment core samples to understand the mat’s development.Figure 1. Locations of the different sinkholes near the MIS area in Lake Huron, MI.
Figure 3. A box and whisker plot of C:N ratios for Middle Island Sinkhole, Lake Huron,
and El Cajon.
Figure 2. 2a (left figure) shows a tray of loaded tin capsules of MIS sediment
core samples. 2b (right figure) shows the Costech Combustible Elemental Analyz-
er (EA) where the tin capsules were analyzed for carbon and nitrogen ratios.
Figure 4. Percentage of organic carbon and nitrogen in Middle Island Sink-
hole, Lake Huron, and El Cajon.
% Carbonorg