George L. Kennedy

The last interglacial period on the Pacific Coast of North America: Timing and paleoclimate

New, high-precision U-series ages of solitary corals ( Balanophyllia elegans ) coupled with molluscan faunal data from marine terraces on the Pacific Coast of North America yield information about the timing and warmth of the last interglacial sea-level highstand. Balanophyllia elegans takes up U in isotopic equilibrium with seawater during growth and shortly after death. Corals from the second terrace on San Clemente Island (offshore southern California), the third terrace on Punta Banda (on the Pacific Coast of northern Baja California), and the Discovery Point Formation on Isla de Guadalupe (in the Pacific Ocean offshore Baja California) date to the peak of the last interglacial period and have U-series ages ranging from ca. 123 to 114 ka. The first terrace on Punta Banda has corals with ages ranging from ca. 83 to 80 ka, which corresponds to a sea-level highstand formed in the late last interglacial period. U-series analyses of corals from the Cayucos terrace (central California) and the Nestor terrace at Point Loma (southern California) show that these fossils have evidence of open-system history, similar to what has been reported by other workers for the same localities. Nevertheless, a model of continuous, secondary U and Th uptake shows that two ages of corals are likely present at these localities, representing the ca. 105 and ca. 120 ka sea-level highstands reported elsewhere. U-series ages of last interglacial corals from the Pacific Coast overlap with, but are on average younger than the ages of corals from Barbados, the Bahamas, and Hawaii. This age difference is explained by the nature of the geomorphic response to sea-level change: fringing or barrier reefs on low- latitude coastlines have an accretionary growth style that keeps pace with rising sea level, whether on a tectonically rising or stable coastline. In contrast, midlatitude, high-energy coastlines are sites of platform cutting during the early part of a sea-level high stand and terrace scouring and concomitant sediment and fossil deposition as sea level starts to recede. The youngest ages of corals from the Pacific Coast suggest that sea level was still relatively high at ca. 116 ka, which is not in agreement with other estimates of relatively large global ice volume at that time. Reliably dated, ca. 120 ka marine-terrace deposits on the Pacific Coast have fossil mollusks that indicate water temperatures as warm or warmer than at present. In contrast, ca. 80 ka marine deposits reported here and elsewhere have fossil mollusks indicating cooler-than-modern water temperatures. The presence of both ca. 105 ka and ca. 120 ka corals on the Nestor and Cayucos terraces explains a previously enigmatic mixture of warm-water and cool-water mollusks. At ca. 105 ka, a relatively high sea level with cool waters may have “captured” the terrace formed during the 120 ka sea-level highstand, in areas of low uplift rate. The inference of cooler-than-modern waters off the Pacific Coast of North America at ca. 80 ka and ca. 105 ka, based on marine-terrace faunas, does not agree with estimates of sea-surface temperatures derived from alkenone studies in the Santa Barbara Basin. However, cooler water temperatures at these times are in agreement with paleotemperature estimates from planktonic foraminiferal data for the Santa Barbara Basin. All records, from central California to Baja California, whether from marine terraces or offshore cores, indicate at least seasonably warmer-than-modern waters during the peak of the last interglacial period at ca. 120 ka.

Late quaternary uplift rates of marine terraces on the Pacific coast of North America, southern Oregon to Baja California sur

Abstract Marine terraces are common landforms along the Pacific coast of North America from southern Oregon to southern Baja California. We report 60 new uranium-series ages of marine terrace corals, hydrocorals, and other fossils. Terraces representing the ca. 80 ka and ca. 125 ka high stands of sea are the most common along the Pacific coast and we have used these ages, along with terrace shoreline angle elevations and paleo-sea-level estimates, to calculate Late Quaternary uplift rates. Uplift rates in the forearc region of the Cascadia subduction zone range from 0.45–1.08 m/kyr. Farther south, along most of the coast west of the San Andreas fault zone, uplift rates range from 0.15–0.35 m/kyr, but some areas have significantly higher rates of uplift and others have apparently experienced little or no uplift in the last 125 ka. In Baja California Sur, west of the East Pacific Rise, little or no uplift has occurred during the Late Quaternary. On average, the magnitude of rates of uplift for the tectonic regimes is as follows: Cascadia subduction zone > San Andreas fault zone region > East Pacific Rise region.

Late Quaternary uplift and earthquake potential of the San Joaquin Hills, southern Los Angeles basin, California

Analysis of emergent marine terraces in the San Joaquin Hills, California, and 230 Th dating of solitary corals from the lowest terraces reveal that the San Joaquin Hills have risen at a rate of 0.21–0.27 m/k.y. during the past 122 k.y. Movement on a blind thrust fault in the southern Los Angeles basin has uplifted the San Joaquin Hills and has the potential to generate an M w 7.3 earthquake within this densely populated area. Our structural modeling suggests that the fault dips to the southwest and slips at ∼0.42–0.79 m/k.y., yielding an estimated minimum average recurrence interval of ∼1650–3100 yr for moderate-sized earthquakes. Recognition of this blind thrust extends the known area of active blind thrusts and fault-related folding southward from Los Angeles into coastal Orange County.

Elemental and isotopic proxies of paleotemperature and paleosalinity: Climate reconstruction of the marginal northeast Pacific ca. 80 ka

Isotope (δ 18 O) and minor element (Mg/Ca, Sr/Ca) analyses of a fossil shell of the marine mussel Mytilus trossulus (Gould) from the Whisky Run terrace at Coquille Point, Bandon, Oregon, enable reconstruction of a paleoclimatic profile for the temperate northeastern Pacific ca. 80–85 ka. The Whisky Run terrace is correlative with marine oxygen isotope (δ 18 O) substage 5a, the final sea-level highstand of the last interglacial complex. Herein we provide sea-surface temperature estimates and report on a new technique that allows estimates to be made of δ 18 O of seawater and riverine discharge from the skeletal chemistry of a single mussel shell. Estimates of mean annual temperature from Mg/Ca ratios are lower than those suggested by the accompanying fossil fauna, and significantly lower than present-day conditions. The data also suggest a decrease in the range of seasonal temperature extremes at 80 ka compared to today. Estimates of the fresh-water and seawater δ 18 O end members, −4.5‰ and +0.5‰, respectively, are slightly greater than modern values.

An evaluation of uranium-series dating of fossil Echinoids from southern California Pleistocene marine terraces

Abstract Fossil sea urchins ( Strongylocentrotus ) from Pleistocene marine terraces on the southern California Channel Islands have been dated by the uranium-series method in order to test the suitability of echinoids for dating marine terraces. Results indicate that urchin plates and spines do not behave as closed systems with respect to both uranium and thorium. Calculated ages based on these data do not agree with uranium-series ages (120,000 and 127,000 yrs) obtained previously from corals from the same localities. Thus, fossil sea urchins ( Strongylocentrotus ) are not considered suitable for uraniumseries dating of Pleistocene marine terrace deposits.