Week 4 (09/15/2009) Summary - Nidhi Patel
Topic: Biogeochemistry and Climate, Hydrates and Greenhouse Gases
Papers Discussed:
Kennett, J. P., Cannariato, K. G., Hendy, I. L., and Behl, R. J.., 2000, Carbon isotopic evidence for methane
hydrate instability during Quaternary interstadials. Science, v. 268, pp. 128-133.
Kohfeld, K. E., La Quere, C., Harrison, S. P., and Anderson, R. F., 2005, role of marine biology in glacialinterglacial CO2 cycles. Science, v. 308, pp. 74-78. (Also, the This Week in Science blurb – “Marine Biology and
Climate” – from page 13 of the same issue.)
McElwain, J. C., Wade-Murphy, J., and Hesselbo, S. P., 2005, Changes in carbon dioxide during an oceanic
anoxic event linked to intrusion into Gondwana coals. Nature, v. 435, pp. 479-482.
Jickells, T. D. et al., 2005, Global iron connections between desert dust, ocean biogeography, and climate.
Science, v. 308, pp. 67-71.
Schmittner, A. and Galbraith, E. D., 2008, Glacial greenhouse gas fluctuations controlled by ocean circulation
changes. Nature, v. 256, pp. 373-376.
Role of Marine Biology in Glacial-Interglacial CO2 cycles – Kohfeld et. al., 2005
This research paper summarizes the role of the marine biological pump as a mechanism to lower atmospheric carbon
dioxide. According to ocean circulation models, physical mechanisms are accountable for at least half of the uptake of
CO2 during glaciations so therefore marine biology has to be responsible for some of the CO2 uptake. The biological pump
dominates carbon and controls it in the ocean. Two main factors needed to enhance marine biological productivity are UV
light and nutrients such as phosphorus, nitrogen and iron. Bottom waters tend to have more oxygen and carbon dioxide
which results in a low 13 C values. In this paper, Kohfeld looks into the changes in CO2 concentrations between glacial
and interglacial periods. Some processes by which marine biota can lower atmospheric CO2 concentrations are via iron
fertilization, whole-ocean nutrient increase, nutrient utilization, CaCO3/Corg rain ratio and silica leakage. More information
about the processes such as the oceanic regions influenced and sediment impact can be found in Table 1. Because this
study hypothesizes an increase in nutrients leading to a decrease in atmospheric CO2 one can use it to analyze/solve future
problems and assess questions from the past dealing with fluctuating CO2 concentrations.
Global Iron Connections Between Desert Dust, Ocean Biogeochemistry, and Climate – Jickells et al., 2005
This research focuses on the iron cycle and its impact on ocean biogeochemistry and climate. The iron cycle which is
shown in Figure 1 has four main factors: the dust availability, atmospheric aerosol loading, marine productivity and the
climate state. Iron, which is essential for all organisms, is a limiting factor for phytoplankton growth because of its ability
to be insoluble under oxidizing conditions above pH 4. This can have a large effect on the global climate system via a
positive feedback or a negative feedback. The main input of iron into the open ocean is aeolian dust transport from the
deserts of Africa and Asia. Other iron suppliers are volcanic sources, anthropogenic sources and extraterrestrial sources.
Dust fluxes can be monitored from direct measurements, satellite observations and extrapolation models and can vary
from glaciation periods to interglaciation periods. Dust tends to increase during glaciation due to aridity, lowered
sea level, changes in vegetative cover and reduced precipitation. Table 2 shows the effect of dust/iron on ocean
biogeochemistry. The oceans have a large impact on climate by means of heat transport and other processes, therefore the
change in ocean circulation will also impact iron transport which will change ocean productivity, oceanic CO2 uptake and
much more.
Carbon Isotopic Evidence for Methane Hydrate Instability During Quaternary Interstadials – Kennett et. al., 2000
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The main goal of this study is to look at instabilities in methane hydrates during the Quaternary interstadials. CH4
hydrates are formed via water under environments consisting of high pressure, low temperatures and abundant gas
concentrations. The hypothesis of the paper is that CH4 outgassing and periodic gas hydrate dissociations were caused by
intermediate water temperature changes associated with the Dansgaard-Oeschger cycles. The authors looked at the
oscillation of 13 C values and 18 O in planktonic and benthic foraminiferas as shown in Figure 2. The records from the
study site, Santa Barbara Basin (Figure 1) showed four occurrences of methane releases which resulted in negative 13 C
changes. More negative 13 C values indicate more productivity, therefore interstadial periods; more positive values
indicate less diversity and less photosynthesis, therefore stadial periods. As temperature goes up, gas solubility goes
down. Gas hydrates which were destabilized set off methane fluxes in the sediments via warm intermediate waters during
interstadials. The deroofing of methane hydrates caused an increase in the amount of CH4 in the water column. An
expanded sulfate reduction zone was present during stadials when cool, intermediate waters stabilized gas hydrates and
reduced the methane flux. Because of a short residence time of 10 years, there is a lack of evidence for the increased
methane. This study analyzes the causes of episodic methane fluxes via the physical, chemical and biological aspects of
the ocean and the relationships between each factors.
Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals. – McElwain et al.,
2005
This study focuses on episodes of enhanced organic carbon burial known as ocean anoxic events. The causes for
these events which lower atmospheric carbon dioxide are unknown. It is hypothesized that during the Toarcian ocean
anoxic event high atmospheric carbon dioxide levels were produced by oxidation of methane released from gas hydrates
or magma-intruded organic-rich rocks. CH4 and CO2 are greenhouse gases and CH4 in the atmosphere oxidizes into CO2
and water. During the event the carbon dioxide was drawn down by 350 p.p.m.v (2.5 C), indicating a global cooling and
then elevated by 1,200 p.p.m.v (6.5 C), indicating a global warming. The study was tested by using fossil leaf stomatal
index, the ratio of the number of stomata to the total number of epidermal cells plus stomata within a given leaf area
expressed as a percentage. The study concluded that magma intrusion was responsible for the event and that light carbon
from the release of methane occurred via intrusion of Gondwana coals by Toarcian-aged dolerites. The evidence shows
that the Toarcian ocean anoxic event was a period of increased warmth preceded by organic carbon burial which is related
to global cooling. The study can help improve ocean-atmospheric modeling, analyze past events and predict future
incidents.
Glacial greenhouse-gas fluctuations controlled by ocean circulation changes – Schmittner and Galbraith
The goal of this study was to focus on the variations on Earth’s climate from the past glacial periods and the
fluctuations on atmospheric greenhouse gases, CO2 and N2O. The study concluded that warming events in
Greenland/North Atlantic were followed by cooling events and are connected with nitrous oxide; Antarctic temperatures
were cool followed by warmer temperatures and are connected to carbon dioxide. The authors focused on climate models
that dealt with changes in the Atlantic meridional overturning circulation and its relationship to the glacial climate and
biogeochemical cycles. One can see the different model simulations dealing with 700 years, 1,100 years and 1,700 years
in Figure 1. Figure 2 shows the temperature changes in Greenland and Antarctica along with the changes in CO2 and N2O;
a close correlation is shown between Greenland- N2O and Antarctica- CO2. The release of carbon from the deep ocean is
caused by an increase in partial pressure of CO2 after the interstadial-stadial shift. The stadial decrease of N2O is
controlled by the upper ocean oxygen cycles. This research can help solve future questions about the atmospheric
greenhouse gases and their relationship to the Atlantic meridional overturning circulation.
SUMMARY
“Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals” by McElwain is
similar to Kennet’s paper, “Carbon isotopic evidence for methane hydrate instability during quaternary interstadials”.
Both of these papers focus on the greenhouse gas methane, more specifically methane hydrates. Methane hydrates which
are also called methane clathrates are CH4 structures in water/ice. Peat deposits/wetlands are a source of methane as well
as sinks for CO2. Therefore, methane in the atmosphere can be changed via changing the amount of wetlands. Methane
has a short residence time of about 10 years in the atmosphere. CH4 and CO2 are both greenhouse gases, so increasing the
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amount of these two gases in the atmosphere causes warming of the oceans. Methane in the atmosphere oxidizes into
carbon dioxide plus water. As shown by these papers, more productivity entails a more negative 13 C value and a low 18
O ratio is correlated with a warmer period. Schmittner’s study also deals with greenhouse gases, carbon dioxide and
nitrous oxide; N2O is not used biologically as opposed to CO2 which is used biologically. Schmittner uses a model to
relate the two different hemispheres to the CO2 and N2O fluctuations on millennial time scales. Kennet tested his
hypothesis using benthic and planktonic foraminiferas as opposed to McElwain who tested her hypothesis using fossil leaf
stomatas.
“Role of marine biology in glacial-interglacial CO2” by Kohfeld and “Global iron connections between desert dust,
ocean biogeochemistry, and climate” by Jickells have many similarities. Both of these papers combine physical,
biological and chemical processes and relate them to climate and the ocean. The main focus of these papers is on iron;
Kohfeld’s study deals with an increase in iron fertilization and its effect on increased marine productivity and Jickell’s
study deals with the effects of iron dust. Global iron dust connections along with correlations of positive and negative
variables is shown in Figure 1 of Jickell’s paper. Older/deeper/colder waters tend to have higher gas solubility, therefore a
higher oxygen and carbon dioxide content as opposed to younger/warmer surface waters. Kohfeld looks at the decrease of
CO2 with respect to an increase in nutrients/iron. Will an anthropogenic fertilization of our oceans with iron decrease our
carbon dioxide content?
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