German Mora Research Interests
My research activities focus on understanding the effects of climatic,
biological, and geological processes on the biogeochemical cycles
of major elements (C, O, H), and on applying and developing proxies
for paleoenvironmental conditions. To pursue these topics, I study
stable-isotope systematics, relying on field experiments because the
partitioning of isotopes in sediments, organic matter, and water depends
on environmental parameters and on the extent of diagenetic and biological
processes. In general, my use of stable isotopes includes two main
themes. One deals with understanding modern environmental processes
using stable isotopes as natural tracers. The second includes the
application of known modern environmental processes to reconstruct
past climates. The most important projects describing my research
include the following:
Assessment of the role of land plants in global change.
This work integrates field measurements of oxygen and hydrogen isotopic
compositions of rain, soil water, groundwater, and plant water in
different ecosystems. One research project focused on deciduous forests
located along a precipitation gradient in the southeastern United
States. In collaboration with Dr. A. Hope Jahren (Johns Hopkins University),
we found that elevated hydrogen isotopic composition of leaf water
occurs in the majority of the studied species early in the growing
season. These high isotopic values occurred irrespective of plant
size, humidity levels, rainfall patterns, or temperature. We concluded,
therefore, that a vital effect related to plant growth induced these
high isotopic values. Based on these interpretations, we also concluded
that a superior substrate for reconstructing land climate is cellulose
from aquatic plants. We are currently investigating this aspect by
studying mosses (Sphagnum sp.) from swales around Lake Superior. Our
goal is to evaluate whether the oxygen and hydrogen isotopic composition
of moss cellulose faithfully records the isotopic composition of rainwater
or groundwater. If our hypothesis is true, then we will be able to
use this technique to reconstruct past climates using peat deposits.
Assessment of the role of terrestrial ecosystems in the modern
carbon cycle.
One of the most intractable questions in terrestrial ecosystem
studies is that of the effect of belowground processes on carbon cycling.
Increasing global warming is likely to stimulate both organic matter
decomposition (i.e., CO2 production) and plant growth (i.e., CO2 assimilation).
If warmer temperatures increase decay rates of organic matter faster
than they stimulate growth rates of plants, then they will cause atmospheric
CO2 levels to increase. The overall objective of our research is to
distinguish root system processes from soil organic carbon (SOC) decomposition
using stable carbon isotopes. In collaboration with a soil ecologist,
Dr. James Raich, EEOB, we are making use of an experimental farm where
C3 crops were replaced by C4 plants. Given that these two plants have
distinct carbon-isotope ratios, we will be able to assess the relative
contribution of root-respired and SOC-derived CO2. These assessments
will be compared with in situ measurements of soil CO2 emissions,
temperature, and soil moisture to evaluate the influence of temperature
and rainfall on belowground CO2 emissions.
Analysis of fossil vascular plant-land tissue to infer paleoenvironmental
conditions.
This work is based on studies of the carbon-isotope systematics of
C3 plants, establishing that the carbon isotopic composition of atmospheric
CO2 can be inferred from sufficient carbon isotope values of vascular
land plant tissue isolates. The method, tested in modern environments,
can be used to interpret the effect of global anoxic-events on the
isotopic composition of CO2. Collaborating with Dr. Hope Jahren at
the Johns Hopkins University, we have determined the carbon-isotopic
composition of cuticles and wood extracted from Lower Cretaceous rocks
of the Potomac Group to investigate the isotopic composition of atmospheric
CO2 during the mid-Aptian anoxic event. We found that there is a prominent
negative excursion in the Arundel Formation that mimics the excursion
found in other terrestrial localities in England and Colombia, providing
further support to the hypothesis that this excursion is global in
nature. My current research effort for this project includes the determination
of the carbon isotopic composition of organic matter and vascular
plant biomarkers (e.g., n-alkanes, isoprenoids, and phenols) in marine
and terrestrial Aptian deposits to evaluate the global nature of this
event.
Quaternary paleoclimatology of tropical regions.
This work addresses the impact of continental glaciations on tropical
regions, particularly addressing the response of terrestrial ecosystems
to changes in atmospheric and oceanic circulation during glacial intervals.
By using carbon isotope values of organic matter present in paleosols
and lake sediments from an alpine basin of Colombia, I documented
the expansion of C4 plants in the northern Andes during Pleistocene.
Two factors can account for this expansion: lowered pCO2 and decreased
rainfall. Although our results were inconclusive, a comparison of
the Vostok ice record with the carbon isotope time-series of lake
sediments from the Bogota basin indicated a possible influence of
pCO2 in the expansion of C4 plants during last 400 kyr. However, our
data of the distribution of sulfur species indicate decreased rainfall
at the same time, which was also corroborated by our oxygen and hydrogen-isotopic
data of pedogenic kaolinite present in paleosols. One possible explanation
for reduced rainfall is a southward shift in the Intertropical Convergence
Zone (ITCZ). To test this hypothesis, we are using a 400 kyr-long
sedimentary record from the Caribbean Sea. We are determining elemental
ratios (Ti/Al, K/Al) in bulk sediments as indicators of rainfall intensity
for northern South America.
Fate of terrestrial organic carbon in marine and estuarine settings.
This long-term project is based on the quantification and chemical
characterization of sedimentary organic carbon and its carbon-isotopic
composition to understand the role of terrestrial carbon burial on
global biogeochemical cycles. I used this methodology in sediments
of the Oyashio Current, finding little accumulated vascular plant
material in these sediments throughout the Neogene.
Although there is some consensus that most terrestrial organic carbon
is mineralized or sequestered before reaching marine sediments, modern
anthropogenic activities have significantly altered the cycling of
terrestrial organic carbon. In particular, these effects of anthropogenic
activities on estuaries are not fully understood. For that reason,
one of my goals is to assess the impact of the fragmentation of natural
landscapes produced by agriculture and residential development on
estuarine carbon cycling. In collaboration with Dr. Donna Surge, we
are addressing this research question by studying four estuaries in
Florida that experience different levels of disturbance. The underlying
hypothesis is that there is a direct correlation between the intensity
of agriculture and the delivery of nutrients to coastal environments.
Therefore enhanced estuarine primary productivity is expected in estuaries
affected by intense agricultural activities. I am using three different
approaches to test this assertion: (1) tracing the flow of nitrogen
released by fertilizers in the proposed estuaries, (2) assessing productivity
levels in the region, and (3) evaluating the current level of disturbance
through an assessment of estuarine metabolic states.
