TABLE 2THE GEOLOGIC TIME SCALE (IN MILLIONS OF YEARS)
The Sierras were first uplifted beginning about 80 million years ago and subsequently eroded into an undulating plain that rose gradually 50 million years ago to Tibet-like heights in east-central Nevada. The present Sierra Nevada includes remnants of this old surface as well as younger granitic rocks that intruded as subduction occurred in the Coast Ranges. A second period of more rapid uplift of the Sierra Nevada began around 3 million years ago. The Klamaths either represent the northern end of the Sierra offset westward by a fault 130 million years ago or are the remains of oceanic terranes lying west of the northern continuation of the Sierra Nevada. During much of the Eocene, 56 to 34 million years ago, the Klamath region was an island with a stable land surface that eroded in a tropical climate, and much of this land surface still exists. The uplift of the Coast Ranges began in Southern California around 30 million years ago and migrated northward with the north end of the San Andreas fault and the Mendocino Triple Junction, where it currently continues. The Coast Ranges began as an offshore submerged subduction complex but were fully joined to the continent by 5 million years ago. The east-west Transverse Ranges arose at around that time because of a bend in the San Andreas fault that resulted in compression during the northward movement of the Pacific plate along the fault.
The Central Valley is one of the largest and flattest valleys in the world. It is believed to have been created when a large slab of oceanic plate was thrust over the North American continental margin during the collison of North America with a west-dipping subduction zone. A new east-dipping subduction zone formed west of this slab in the modern Coast Ranges, leaving a broad gap in between. It lay beneath ocean until about 5 to 15 million years ago depending on location. Between about 5 and 2 million years ago, mountain uplift created a marine embayment in the Central Valley encircled by mountains and draining to the ocean from the southern valley near Monterey Bay. Continued mountain uplift later blocked this seaway, and by about 600,000 years ago the Central Valley was a freshwater basin draining through the region of present-day San Francisco Bay (Harden 2004).
Deserts in southeasternmost California contain ancient continental rocks that attest to their having been part of ancient North America rather than accreting onto the edge, as did most of the west coast. The deserts also harbor more extensive fossils of ancient terrestrial life than the rest of the state, including Eocene terrestrial vertebrates. However, their history as deserts is quite recent (see below).
The California Channel Islands are seafloor ridges, transported northward, rotated, and uplifted by movement on the San Andreas fault beginning 28 million years ago. Although the four smaller islands were inundated at various times in the Pleistocene, beginning 1.8 million years ago, parts of the four largest islands have been continuously exposed for at least 600,000 years, and it is even possible that they had ancient (Miocene) connections to the continent. During the lower sea levels of the Pleistocene, the four northern islands were united into the superisland of Santa Rosa lying only 6 kilometers from the mainland. Santa Cruz, the closest island today, is now 30 kilometers from shore.
CLIMATIC HISTORY
The global climate has generally cooled and dried over the past 50 million years, and plate tectonics have played an important role by altering oceanic circulation and atmospheric composition. Major climate-changing events have included the opening and widening of Antarctic ocean passageways, the uplift of the Himalayas and consequent decline in atmospheric CO2 due to chemical weathering, and the arrival from the west of Panama and closure of the Central American seaway. Superimposed on these longer-term trends are oscillations on the order of tens to a few hundreds of thousands of years, caused by variation in the eccentricity, obliquity, and precession of the earth’s orbit around the sun (Milankovitch cycles). The interaction of these forces is complex. In particular, it appears that the tectonically driven changes have increased the sensitivity of the climate system to orbital forcing, leading to increasingly rapid and extreme climatic fluctuations toward the present day (Zachos et al. 2001).
The earth’s transition “from greenhouse to icehouse” has been reconstructed mainly from the carbon and oxygen isotopic composition of foraminiferan shells recovered from Antarctic deep-sea drilling (Figure 10). Warming trends from the mid-Paleocene (59 Ma) to early Eocene (52–50 Ma) produced the Eocene climatic optimum, a time when much of the earth experienced climates resembling today’s wet tropics, except for caps of temperate climate at the poles. This was followed by a long period of cooling, leading to the formation of Antarctic ice sheets by the early Oligocene (34 Ma). Moderate warming from the late Oligocene (26 Ma) to the middle Miocene (15 Ma) reduced the extent of oceanic ice, although temperatures never regained their Eocene levels. Cooling resumed from the middle Miocene to the middle Pliocene (6 Ma). The Northern Hemisphere Glaciation, beginning 3.2 million years ago, marked the beginning of extreme oscillations, with more than 20 relatively long glacial periods interrupted by shorter interglacials (Figure 10; Zachos et al. 2001). The present interglacial period began at the Pleistocene-Holocene boundary 12,000 years ago and is expected to end with another ice age unless disrupted by anthropogenic additions of greenhouse gases.
Today’s five mediterranean climate regions with their characteristic winter rainfall and summer drought are found on west coasts between roughly 30° and 42°; latitude (Figures 2, 3). In these locations, the jet stream brings winter storm systems from maritime rather than interior sources, leading to cool, rainy winters instead of cold, snowy ones. The high-pressure systems that create the world’s major deserts (from 23° to 30° latitude) shift poleward in summer with the earth’s tilt, creating the desertlike summer drought. Upwellings of cold deep-ocean water along the coast, which produce a marine layer of cool air capped by warmer inland air, are also a key ingredient of both the summer drought and the relatively gentle winter weather. Timing and duration of the summer droughts vary considerably among the five world regions, with California’s being among the longest and driest (Dallman 1998).
FIGURE 10. Global relative temperatures from the Paleocene to the present, based on oxygen isotope measurements from deep-sea sediments and ice cores. (Based on data from J. Zachos; see also Zachos et al. 2008)
Because of its relevance to plant and animal evolution, the history of the mediterranean climate is of great interest. In today’s mediterranean zones, tropical-like climates began to give way to more seasonal ones as many as 40 million years ago. Precipitation began to peak in the winter by the middle Miocene. However, the fully mediterranean climate with its near-complete summer drought emerged only after the onset of the Pleistocene brought the development of cold offshore currents. While glaciation in the Northern Hemisphere was well under way 2.7 million years ago, the modern ocean current system was not in place until about 1.5 million years ago (Ravelo and Wara 2004). The uplift of the state’s major mountain ranges in the past 5 million years contributed to increasingly steep internal climate gradients. Based on plant fossil evidence, Raven and Axelrod (1978) argued that some summer rainfall persisted in California until one million years ago, but this has yet to be corroborated with geophysical evidence.
Another important question is how severely the climate fluctuated during glacial-interglacial cycles. The conventional wisdom, largely from plant-based evidence (see later sections of this chapter), is that the climate was colder and rainier but remained mediterranean during glacial periods. However, isotopes indicate that during the coldest parts of the last several glacial periods, expanded oceanic ice sheets blocked the cold oceanic current system, producing warmer and rainier conditions in California resembling a prolonged El Niño. Fossil pollen indicates that while this advance warming speeded the recovery of the interglacial vegetation, it caused declines in abundance of the fog-dependent coastal redwoods (Herbert et al. 2001).
FLORISTIC HISTORY
Origin of the Flora According to Raven and Axelrod
Building on their decades of research in plant evolutionary biology and paleobotany, respectively, Raven and Axelrod (1978) described