Based on pollen records, plant macrofossils, vertebrate paleontology, noble gas temperatures from ground water, Sr/Ca isotope data, and carbon and oxygen isotopes from speleothem calcite, dramatic climatic changes have occurred in the Gulf Coast over the past 20,000 to 35,000 ybp. Toomey et al. (1993) provide an excellent compilation of paleoclimatic indicators in models that recognize six climatic periods characterized by different temperature or moisture levels.
- full glacial (20-14,000 ybp) - cooler temperatures (cooler than the modern by at least 6( C) and higher moisture levels (much higher than at any time since)
- late glacial (14 - 10,500 ybp) - rapid temperature increase (between 15,000 and 13,000 ybp, temperatures were within 2 to 3( C of modern levels) and then, an effective moisture decrease, then increase.
- early to middle Holocene (10,500 - 5000 ybp) - decreased effective moisture levels
- late Holocene I (ca. 5000 - 2500 ybp) and late Holocene II (ca. 2500 - 1000 ybp) - Holocene I was initially dry, while by the end of Holocene II, is was more moist
- modern (ca. 1000 ybp to present) - little available data for the most recent time period. Toomey et al. (1993) observe a return shift to moist conditions, while Delacourt and Delacourt (1977) do not see moist conditions after the establishment of modern flora.
The relationship between paleoclimatic indicators and global climate models is examined to determine if the models accurately recreate regionally observed climatic trends. If detailed paleoclimatic information is not available for a region, can the climatic shifts suggested by global climate models be reliably integrated into studies trying to link climate changes and other factors? Toomey et al. (1993) compared empirical paleoclimatic results with those predicted by global paleoclimate modeling experiments conducted by COHMAP (1988). Based on these results, global climate modeling results can be used to reflect major climatic trends, but caution should be used when linking trends to absolute dates and when observing climatic occurrence with short time scales.
Paleoclimatic indicators were combined with geologic data to demonstrate the effect of temperature and moisture on vegetation type and coverage and the soil profile.
- full glacial - cool and moist conditions supported complete vegetation cover with deeply weathered soil profiles
- late glacial - cool and moist conditions supported complete vegetation cover with deeply weathered soil profiles
- early to middle Holocene - increased temperature and decreased moisture led to diminished vegetation cover, decreased soil production, and increased erosion rates resulting in thinner, but stonier soils
- Holocene I - extremely dry conditions caused a shift in vegetation to short grasses and scrub. Decreased soil production and increased erosion rates led to the near complete removal of remaining soil
During the evolution of the depositional environments of the Rio Grande system, there were periods in which sediment supply overwhelmed eustasy. During the same time intervals, several terrestrial climatic fluctuations occurred.
- 19,000 to 5,000 ybp - sediment supply increased.
- full glacial - cooler temperatures and higher moisture levels encourage development of dense vegetation and thick soil profiles. Although dense vegetation is evident, the Rio Grande system does record a huge influx of sediment.
- late glacial - rapid temperature increase and a moisture decrease followed by an increase led to a decrease in vegetation cover. The later increase in moisture could cause more precipitation and transport some of the exposed soil into the fluvial system.
- middle Holocene - decreased effective moisture levels occurred causing a decrease in vegetation and increase in erosion of soils.
- Once sediment enters the fluvial system, what determines if it is sequestered, carried downstream to the shelf, or both? According to Blum et al. (1994), storage occurs when sediment supply exceeds power and removal or bedrock incision when the opposite occurs.
- 20,000 to 14,000 ybp - characterized by channel aggradation and sediment storage in the fluvial systems. Although the rivers were storing sediments, the huge increase in sediment yield on the shelf between the highstand and lowstand system tracts indicates that a large amount of sediment was being carried downstream.
- 14,000 to 11,000 ybp - channel incision in the fluvial systems observed, meaning there was greater stream capacity than sediment load. As an increase in sediment supply is observed on the shelf, the effect of incision in a fluvial system is interpreted as causing the movement of greater amounts of sediment onto the shelf.
- 11,000 to 5000 ybp - increased valley fill deposition. Sediment supply to the shelf is large during this time period; therefore, the fluvial systems, while storing sediment, must have been moving large amounts of it onto the shelf.
- 5000 ybp - at the same time as a switch from deltaic to offshore marine deposition occurred on the shelf, a switch to very dry conditions occurs in the drainage basin. Moisture is a critical factor in sediment supply is also supported by the fact that sediment yield was greatest during the highest moisture conditions in the drainage basin.
- The climate and sediment supply relationship during HST 1 and HST 2
The relationship between cooler and moister climatic conditions and higher sediment supply suggested by this study is demonstrated.
Modern Rio Grande system sediment supply
Examination of suspended and bed load values from the upper Rio Grande drainage basin led to an estimate of the volume of sediment carried by the Rio Grande system. Using a total annual sediment load of 25.8 million tones, the calculation of a 1000 year sediment load results in a value of approximately 22 km3.
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