Geography 40
Global Environmental Change
Fall 2002



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Lecture 24.
The Historical Geography & Biogeography of Tidal Salt Marshes
Key words: Geography, Biogeography: The Extent & Distribution of Marshlands (Distinctive Landscapes & Ecosystems)
The Extent & Distribution of Marsh Organisms
Historical
Past, Present, and Future
Tidal Marshes vs. Salt Marshes

Slide 1: Sea level fluctuations on varying timescales. In the last 20,000 years sea level has risen approximately 120 m.

Slide 2: cycles of the Quaternary. O18 values of ocean sediments are an index of global volume of water locked up in ice-sheets. These data show oscillations between glacial advances and retreats (interglacial periods) occurring on approximately 100,000 year cycles throughout the Quaternary period.

Slide 3: Modern distribution of tidal marsh habitat. Primarily found in midlatitudes.

Slide 4: Sea level and coastline changes: coastline changes related to the glacial and deglacial cycles determine the extent, distribution and connectedness of tidal marshes. Impacts of the climate cycle include: raising and lowering of sea level; isostatic rebound (the crust rebounds after ice weight has been removed – over long period of time); changes in the latitudinal distribution of marsh habitat due to temperature gradient; fresh water flows related to changes in precipitation patterns; and sediment supply and transport, related to these fresh water flows.

Conditions for tidal marsh development:
Protected shoreline (from high energy waves and storms)
Sufficient sediment supply
Genetic material (marsh-adapted plants)

Constraints on plant life in a tidal salt marsh:
Variable environmental conditions
Periodic inundation
Salinity and osmotic stress
Anoxic and reducing conditions
Population Isolation
These constraints can be summarized by considering the daily cycle of tidal inundation:
Slide 5

Adaptations to life in a tidal salt marsh:
Ability to change morphology to fit environmental conditions (Phenotypic plasticity). E.g., some plants can grow very tall if need be, but this is not a genetic change
Ability to transport adequate oxygen supplies to plant tissues, with aerynchema
Ability to produce propagules that can be dispersed in water
Ability to complete life cycle during optimal environmental conditions (annual habit)
Ability to excrete or filter out excess salts
C4 photosynthetic pathway which allows for more efficient water use and CO2 intake


Plant zonation in tide marshes: related to hydroperiod (the frequency, depth and duration of tidal flooding) which is determined generally by elevation and/or distance from channels.
Slide 6: zonation
Slide 7: SF Bay.jpg
The San Francisco Estuary
Drains 40% of California (the 2 rivers, Sacramento and San Joaquin together drain Northern CA and some of Central CA, they meet at the Delta, approximately 50 miles inland from the Golden Gate, and the fresh water flows through the Estuary to the Pacific ocean.
The Bay Estuary is the result of geology: structural constrictions at the Golden Gate and the Carquinez strait allow for shallow, low energy inland bays.
A natural salinity gradient extends from the Golden Gate (marine conditions) to the Delta (fresh water).
Slide 8: SF Bay LGM
San Francisco Estuary did not exist prior to about 15,000 years ago. During the last Glacial Maximum (ca. 20,000 years ago) the Bay was a river system meeting the Pacific out beyond the Farallon islands (which were then the Farallon Ridge).

Slide 9: CA Coastline
To find a modern analogue for the former SF Bay, we can look along the coast of CA at river-delta systems like the Eel River. These deltas have smaller total acreage of tidal marshland than the protected Estuary.
Also note the trench which lies offshore of the North American coastline: this is an Active Continental Margin.

Paleoresearch involves the use of proxy data – data that can indirectly measure conditions that cannot be directly measured. The carbon isotopic value of the organic sediments and the preserved pollen have been used to reconstruct the paleo-salinity history of several marshes in the North Bay.

Slide 10: C isotope.jpg
Carbon exists in three forms (isotopes) on Earth: C14 (unstable, or radioactive); C12 (stable, and most common) and C13 (stable, and rare).
Slide 11: C and PS.jpg
During photosynthesis, plants fractionate carbon isotopes (that is, they preferentially take in more C12 than C13) as they convert C from the atmosphere into sugars. Different plants fractionate more or less against C13. The isotopic value of plants is the ratio of C13 to C12 measured against a standard and given in units of per mil (‰).
Slide 12: PS and 13C.jpg
How much fractionation occurs depends on the Photosynthetic pathway used by the plants. So the C4 photosynthetic pathway fractionates less against C13 and therefore has a higher isotopic value (less negative).

Slide 13: Pollen.jpg
Slide 14: MultiProxy.jpg
Because it is difficult to distinguish between Pickleweed (which is extremely salt tolerant) and other C3 plants (which generally are not salt tolerant) using the isotopic method, pollen is used as an additional proxy method. The pollen grains deposited by plants are well preserved in the sediments and can be identified to the family level. Pickleweed has an easily identified grain.
Slide 15: marsh fm.jpg
In recent years the paleo-history of several marshes along the Estuary have been studied. The minimum ages of establishment of these marshes is given. Note that the site at China Camp has 2 dates, after the first marsh period, the site became subtidal (below sea level) once again 1000 + years, until reestablished again at the later date. Also note the two marshes, Benicia and Peyton, while close in space, have very different dates of establishment. Peyton receives greater sediment supply from Walnut Creek, while the Benicia site has no such creek (that is as large as Walnut Creek).
Slide 16Slide 17 Slide 18: stratigraphy and data from China Camp
These slides show the stratigraphy at the China Camp marsh site. The location of the cores collected from CC are back away from the shore line, close to the drainage off the surrounding hills, which may account for the increased freshwater pollen seen in some places in the core. On the Y-axis of the data sets is the Chronology in calibrated years B.P. (before present, and calibrated with tree ring dates). The X-axes are variable, for the isotope data, the increase in C4 influence is shown from left to right and represents an increase in the salt-tolerant grasses, either Cordgrass (low marsh) or Salt grass (high marsh). The changes in sedimentation rate are worth noting: there are periods of very low sediment accumulation (like the period between about 2500 cal. yr BP. and ca. 800 cal. yr B.P. ) and some very abrupt changes.
The pollen data shows the pollen types (plant names across the top) and their relative percents throughout the core (i.e., percent of total pollen for each sample). Note that periods with higher amounts of Sedge pollen, and relatively low amounts f Pickleweed represent times that may have had saltier conditions.
Slide 19 Slide 20: Sed. rates and Salinity
These slides combine data from several sites along the Northern reaches of the Estuary, that stretch which forms a natural salinity gradient. From left to right, sites are located from the western edge of the Estuary to the Eastern edge, Browns Island, which has the freshest conditions.
Sedimentation rates from all the sites show some similarities, though the exact timing of sedimentation rate changes is difficult to fix.
The changes in salinity are inferred from increased salt tolerant pollen (pollen data has been reduced to normalized index, with increased salt tolerance to the right) and increased presence of C4 grasses.
Slide 21: Summary.jpb
So physical factors (such as sediment supply, salinity of estuarine waters; sea level) play an important role in the formation and maintenance of tidal marshes and on the organisms that occupy the marshes. In addition, historical processes have produced and modified these ecosystems (changes in sea level resulting from glacial cycles, changes in sediment supply related to climate) and continue to work on these ecosystems.

Slide 22: Future
This slide demonstrates again the potential role of glacial-scale processes on marshes. Ca. 14000 years ago, a tremendous pulse of fresh water , possibly from the collapse of part of the Antarctic ice sheet, resulted in a catastrophic rise in sea level of about 20 meters in a matter of decades. This sort of rapid, massive, rise in sea level could be repeated and may even result from anthropogenic global warming.

Lecture 25:
Short-term climate variability and the Holocene

Timescales of Change:
The last major continental glaciation reached its maximum ca. 18,000 bp. The temperature difference between then and today is on the same order of magnitude as the temperature change predicted in response to global warming. The interval from the last glacial retreat (ca. 12,000 years ago) is the Holocene. Looked at from a long term (over 150k) timescale, the Holocene looks uninteresting (fig. 12-1a).
But if we zoom in and look at the last 10,000 years, or last 1000 years, we see that there has been quite a bit of smaller-scale climate change.
There have also been major ecological changes during the Holocene deglaciation. Species found today existed prior to the last glaciation, but the assemblages seen today in specific environments has changed. E.g., assemblage of the boreal forest community in high midlatitudes came into being only during the Holocene.
More importantly, humans societies for the most part evolved during this last Epoch and have had profound effects on the Earth system.

Climate change and variability:
Climate: weather conditions experienced at a location over some reference time frame (30 years is standard).
Climate variability: fluctuations that take place within that interval.
Climate change: significant difference in the average conditions or in the pattern of variability between two timespans. (comparing one 30-year interval to another; or to average of last 100 years, or last 1000 years).
No such thing as "normal" climate – climate varies continuously.
Looking for trends in data. e.g., 12.1d plot of global mean temperatures based on historical records shows general warming trend.
To extend the temperature series further, we need to use Proxy data.
Proxy data: data that cannot be obtained by direct measurement, but can be inferred from other evidence. E.g., last time we talked about changes in tidal marshes – specifically around the S.F. Estuary and used pollen and stable carbon isotopes to infer changes in salinity. We’ll come back later in the semester to more specific discussion about salinity changes in the SF Estuary.
Today we have lots of data about present climate. We use satellite data (especially since the 1970s), as well as daily observations made by people.
The principle of uniformitarianism is critical to the use of proxy data to infer past climate conditions. Processes operating today also operated in the past, and past geologic events can be explained in terms of processes active today.

The most common proxy techniques are
Palynology – pollen and organic micro-fossils preserved in depositional environments, like lakes or marshes (or bogs).
A basic underpinning for using these types of proxy data is that we be able to produce stratigraphies – sedimentary layers that have accumulated over time with the oldest at the bottom and the youngest near the top.
Gives the plant assemblages that lived at past times represented by parts of the core.

Dendrochronology – dating trees by counting the annual growth rings. The width of each ring indicates the amount of growth that occurred during the season, which can be related to temperature or to water availability.
Oldest trees in the world are in the White Mountains of CA, the bristlecone pines (almost 5500 year old record).

The Holocene
Assembling proxy evidence of climate from around the world, the Holocene had a considerable range of climate change and variability.
Dominance of temperature changes in the mid- and high latitudes, whereas the tropics and subtropics appear to have experienced greater changes in moisture.
It is difficult to establish which broad pattern changes during the Holocene were global.
Temporal control and spatial control are difficult. Evidence of climate change may be found at many sites occurring at the same time, but not found everywhere. Or maybe not at the same magnitude.
Consider that: when talking about the "mean global temperature", relatively small changes are associated with relatively large changes in the environment. E.g., the mean global temperatures at the height of the LG were probably only 5 to 7 degrees lower than present. And, 800 years ago, global mean temperatures were only .5 degrees C warmer than today, but the Vikings were able to colonize parts of Greenland and grow crops to maintain a continuous settlement. Global temperature changes predicted for a doubling of CO2 in atmosphere is on the order of 1.5 to 4.0 degrees C. Greater than anything that has occurred in the past 10,000 years and are comparable to the warming that took place between the LGM and today.

The Younger Dryas:
A climatic reversal which occurred about 10,500 years ago. Earth was warming ca. 15000 years ago to about 10,000 years ago. Ice sheets retreating, global sea levels rose, vegetation began to colonize previously glaciated landscapes and new vegetation patterns developed as soils formed and temperatures and rainfall patterns changed.
This gradual spread of warmer conditions came to an abrupt end ca. 12000 – 10,000. (Ruddiman, figure 14-6), preceded by a meltwater pulse (Ruddiman, 14-5).
At this time, pollen data from Northern Europe show abrupt changes in plant assemblages:
As climate warmed after 15,000 years ago, Pattern: there was an increase in vegetation density, especially in grasses and sedges. Also increases in shrubs and willow, and it some areas the shrubs were then replaced by woodland. This pattern was similar in British Isles, Ireland and Scandinavia. The Geologic evidence showed that most of Scotland was deglaciated by 11,000 ybp.
Then a major reversal occurred. Within a short time there was a new ice sheet several 100 meters thick over Western Scotland, and renewed advance of valley glaciers in upland regions of No Europe. Pollen evidence shows synchronous change in vegetation with woodlands diminishing and restricted to a few sites. Vegetation became more open and mostly cold tolerant types, like the Dryas flower.
The evidence of the Younger Dryas event is strong in Northern Europe, indicating a major climate shift there. But global impacts were also felt, though less obvious.
New England, similar climate shift, and along E. coast of Canada.
Gulf of Mexico and Gulf of California of some shift, and limited evidence from the Mediterranean.
Climate reversals also occurred in the Andes and in Africa, where lake levels, which had increased after norther deglaciation, and savannah conditions in the Sahara Desert, then lake levels dropped ca. after 11 ky ago. So while the Y.D. occurred in no. Europe, much of tropical and subtropical Africa was much drier.
Ocean cores from western tropical Pacific and off the coast of Japan show some indication of climate shift as well.
Glaciers in So. Alps of New Zealand may have readvanced as well.
So, Younger Dryas was centered on the Northe Atlantic region, but synchronous changes occurred all over the globe. What could have caused a strong regional change, yet able to influence widely scattered areas?

North Atlantic Deep-Water Formation.
Remember the mild conditions in Northern Europe are due to the warm waters in the northeast North Atlantic – Gulf Stream and No. Atlantic Drift which are partly due to Thermohaline circulation. Also remember that thermohaline circulation is driven by deep water formation in the Norwegian and Greenland Seas.
Wallace Broecker, a geochemist at the Woods Hole oceanographic institute, suggests that some of the changes may have resulted from deep water formation being cut off during deglaciation.
Normally meltwater from the No. American ice sheet flowed southward to the Gulf of Mexico, but might have been blocked by a retreating lobe of ice, the meltwater would then have flowed east through the Gulf of St. Lawrence. This would have brought cold Freshwater to the northern No Atlantic. A Stable surface layer would freeze easily, pushing sea ice margin southward and cut off formation of No. Atlantic deep water. Both the change in thermohaline circulation and the southward expansion of sea ice would have cut off the flow of warm surface water in the No. Atlantic Drift.

On-Off Switch in the North Atlantic. Ice core data from Greenland ice cap shows that changes during the Younger Dryas event may have occurred in less than a decade. Fig. 12-2 shows that snow accumulation occurred during the warm intervals just before and after the YD event. And the change in accumulation occurred in less than a decade. Dust deposited on the ice also reveals rapid changes in rates. More dust in glacials because increase temperature gradient (therefore increased atmospheric circulation). Suggest an almost instant switch between modes of circulation. It isn’t clear why, but may be explained by Chaos theory, which has explained long range weather forecasting is impossible because atmosphere is a chaotic system. Chaotic systems are iterative – the state of a system at one point in time is dependent on the state at previous point. Slight changes in the starting point are amplified through positive feedbacks, so possible results diverge rapidly.

Dating issues for early Holocene: Ruddiman fig. Box 14-1. Deglacial 14C dates are too young compared to Th/U dates. Compared against tree ring counts, and the difference increases around 10,000 years ago. Has to do with the rate of cosmic bombardment. Atoms of 14C result from cosmic particles from space transforming 14N in our atmosphere into radioactive 14C.
Lecture 26:
Short-term climate variability and the Holocene

Recap:

The Younger Dryas:

North Atlantic Deep-Water Formation.
Mild conditions in Northern Europe are due to the warm waters in the northeast North Atlantic – Gulf Stream and No. Atlantic Drift which are partly due to Thermohaline circulation.
Thermohaline circulation is driven by deep water formation in the North Atlantic.

Wallace Broecker, a geochemist at the Woods Hole oceanographic institute, suggests that some of the changes may have resulted from deep water formation being cut off during deglaciation.
Normally meltwater from the No. American ice sheet flowed southward to the Gulf of Mexico, but might have been blocked by a retreating lobe of ice, the meltwater would then have flowed east through the Gulf of St. Lawrence. This would have brought cold Freshwater to the northern No Atlantic. A Stable surface layer would form, inhibiting deep water formation; in addition, the fresher surface layer could freeze easily, pushing sea ice margin southward and cut off formation of No. Atlantic deep water. Both the change in thermohaline circulation and the southward expansion of sea ice would have cut off the flow of warm surface water in the No. Atlantic Drift.

On-Off Switch in the North Atlantic. Ice core data from Greenland ice cap shows that changes during the Younger Dryas event may have occurred in less than a decade. Higher snow accumulation occurred during the warm intervals just before and after the YD event. The change to reduced accumulation occurred in less than a decade.
Other evidence: Dust deposited on the ice also reveals rapid changes in rates. More dust in glacials because increase temperature gradient (therefore increased atmospheric circulation). Suggest an almost instant switch between modes of circulation

Dating issues for early Holocene: Deglacial 14C dates are too young compared to Th/U dates. Compared against tree ring counts, and the difference increases around 10,000 years ago. Has to do with the rate of cosmic bombardment. Atoms of 14C result from cosmic particles from space transforming 14N in our atmosphere into radioactive 14C.

Holocene Climatic Optimum
After the YD event ended, with a rapid shift to warmer conditions, climate remained fairly constant, or had slow warming over the next several thousand years. This Climatic Optimum was from ca. 6000 to 5000 years ago temperatures were slightly higher than today. This interval was fairly stable and mild, particularly in Europe.
Examples from elsewhere around the globe:
* In E. Africa and Sahara desert, conditions were much wetter. Ancient lake levels were higher, probably because of increased evaporation from warmer temperatures. The ITCZ shifted northward in the No. Hemisphere summer, bringing higher rainfall to Sahara and E. Africa.
* In Arabia, enhanced monsoonal circulation in parts of Middle East (Arabia) and NW India.
* The Tarim Basin, where ancient Silk Route from China to Europe, is a desert today, but was forested and had many settlements 5-6000 ybp.
* Settlements and agriculture existed in the Indus river valley (today the Rajasthan Desert) and Nomads were able to graze cattle in the Sahara.
Did climate change lead to the downfall of these settlements or human land use practices? Possibly both.

Medieval Warm Period
After the Holocene Climatic Optimum, temps dropped again, until about 3000 years ago, when conditions again became warmer.
Examples :
* In Greenland, temperatures increased after A.D. 6000 – 650 with temps in the North Atlantic reaching a maximum around 1100 AD. Viking colony on SW coast of Greenland. The colony lasted almost 400 years, but decreasing temperatures increased the sea ice east of Greenland by about 1200 AD. By the middle of the 14th century, ships had to change routes to avoid ice and by AD 1410 communication with Greenland colonies was lost.
* Farther south, in northern, western and central Europe, the MWP reached a maximum between AD 1150 and AD 1300.
Wheat grew in Norway at ca. 64 °N, oats and barley in Iceland, vineyards in England and
* Farm settlements spread to higher elevations in Norway, northern England and Scotland.
* Average temperature of Central England during this period was about .5 to .8°C above the mean for the first half of the 20th century.

At the end of this "MWP", climate conditions became highly variable, with increased storm frequency in the North Atlantic, summers that were wetter and colder, leading to failed harvests and famine throughout Europe and Scandinavia, all around AD 1250 to 1350. The Bubonic Plague, which lasted from AD 1346 to 1361 further devastated societies already in trouble. 25 million people (1/4 Europe’s population) died from the Bubonic Plague.

The Little Ice Age
Started during the late 1500’s – changes towards cooler conditions. Evidence mostly found in western Europe and No. America, but also from around the world including the Alps, Asia, the Himalayas, South America, New Zealand and Antarctica.
Evidence includes:
o re-advance of mountain glaciers (not in ice sheets) E.g., documented records of glacial advance in the Swiss Alps, covering houses and advancing on villages,
o lowering of tree lines (the maximum elevation that trees are found),
o increased erosion and flooding,
o sea-ice expansion, and
o freezing of canals and rivers. E.g., canals of Holland have been used for transportation and so records have been kept for a long time of freezes since 1633. During the LIA, the canals commonly froze for 3 months at a time, while today they seldom freeze for long.

The LIA is similar to the Younger Dryas in that it had a strong regional focus (Europe), but also impacted areas all over the globe.

The changes which may have been due to the LIA that have been found across the globe may not have been synchronous, and duration of change at different places may not have been the same. Evidence is not clear. In general, the LIA lasted from the late 1500s through the mid 1800s. However, the LIA had a lot of variability, with episodic cold spells that varied in timing and duration from place to place.

Unlike the YD, the cause of the LIA does not seem to have anything to do with the retreat of ice sheets.

Possible causes of shorter timescale climate change: Volcanoes and Climate:
Atmospheric composition can change and have an impact on climate. What could cause changes in atmospheric composition on the relatively short timescales of 100s to 1000s of years, such as discussed here? 1. anthropogenic activity; 2. volcanic eruptions. The latter more likely for this earlier period in human history.
Volcanic eruptions put not only ash particles into the atmosphere (which falls out of the atmosphere quickly within 10s to 100s of KM from the volcano), but more importantly, Sulfur dioxide SO2. SO2 injected high into the stratosphere, then oxidizes and forms sulfuric acid droplets, atmospheric aerosol. The aerosol scatters and reflects solar radiation, and also absorbs infrared radiation, warming the stratosphere. These effects are global and last 1 to 2 years. The temperature change associated with volcanic eruptions is on the order of 0.2C to 0.3C for 1 to 2 years after an eruption, but some eruptions can result in cooling of 0.3C to 0.7C lasting 1 to 3 years.

Other causes of Holocene Climate Change
Orbital Changes and Greenhouse variations. Precessional forcings are at 19KY and 23 KY, and seasonal and latitudinal distributions of insolation ca. 18 KYA were very similar to those of today. The atmospheric CO2 concentration was ca. 200 ppm 18 KYA. By the mid Holocene, precession changed in a way that seasonality (diff. between summer insolation and winter insolation) increased in the No. Hemisphere and decreased in the Southern Hemisphere. The atmospheric CO2 concentration increased to about 265 ppm.
General Circulation Models simulated global climate at 3 ky intervals throughout the Holocene and matched well the proxy records.

Solar variability: Sunspots. Sunspot cycles vary, but on average are about 11 years from minimum to minimum. Periods with maximum sunspot activity are periods with slight increase in solar output. The increase is so small, that the mechanism involved in how they would affect climate is not yet known However, comparing the variations in the sunspot cycle length and changes in temperature for the No. Hemisphere over about a 120 year period shows a very close fit.

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