B. Lynn Ingram

Spatial and temporal variability in the stable isotope systematics of modern precipitation in China: Implications for paleoclimate reconstructions

The stable isotopic composition of materials such as glacial ice, tree rings, lake sediments, and speleothems from low-to-mid latitudes contains information about past changes in temperature (T) and precipitation amount (P). However, the transfer functions which link δ18Op to changes in T or P, dδ18Op/dT and dδ18Op/dP, can exhibit significant temporal and spatial variability in these regions. In areas affected by the Southeast Asian monsoon, past variations in δ18O and δD of precipitation have been attributed to variations in monsoon intensity, storm tracks, and/or variations in temperature. Proper interpretation of past δ18Op variations here requires an understanding of these complicated stable isotope systematics. Since temperature and precipitation are positively correlated in China and have opposite effects on δ18Op, it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions. Here, we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord. The strength of these transfer functions in China is investigated using multiple regression analysis of data from 10 sites within the Global Network for Isotopes in Precipitation (GNIP). δ18Op is modeled as a function of both temperature and precipitation. The magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence. δ18Op values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature, and therefore exhibit more negative values in the summer. In contrast, δ18Op values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between δ18Op and temperature. The sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation. Comparison with simple linear models (δ18Op as a function of T or P) and a geographic model (δ18Op as a function of latitude and altitude) shows that the multiple regression model is more successful at reproducing δ18Op values at sites that are strongly influenced by the summer monsoon. The fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally. Since the Southeast Asian monsoon intensity is known to exhibit large variations on a number of timescales (annual to glacial–interglacial), and the magnitude and sign of the transfer functions is related to monsoon intensity, we suggest that as monsoon intensity changes, the magnitude and possibly even the sign of the transfer functions may vary. Therefore, quantitative paleoclimate reconstructions based on δ18Op variations may not be valid.

Reservoir Ages in Eastern Pacific Coastal and Estuarine Waters

We have refined marine reservoir age estimates for eastern Pavific Coastal waters with radiocarbon measurements of mollusk shells collected prior to 1950. We have also investigated interspecific variability in 14C ages for historic and ancient shells from San Francisco Bay.

An archaeological and paleontological chronology for Daisy Cave (CA-SMI-261), San Miguel Island, California.

We provide detailed contextual information on 25 14 C dates for unusually well-preserved archaeological and paleontological remains from Daisy Cave. Paleontological materials, including faunal and floral remains, have been recovered from deposits spanning roughly the past 16,000 yr, while archaeological materials date back to ca. 10,500 BP. Multidisciplinary investigations at the site provide a detailed record of environmental and cultural changes on San Miguel Island during this time period. This record includes evidence for the local or regional extinction of a number of animal species, as well as some of the earliest evidence for the human use of boats and other maritime activities in the Americas. Data from Daisy Cave contribute to a growing body of evidence that Paleoindians had adapted to a wide variety of New World environments prior to 10,000 PB. Analysis of shell-charcoal pairs, along with isotopic analysis of associated marine shells, supports the general validity of marine shell dating, but also provides evidence for temporal fluctuations in the reservoir effect within the Santa Barbara Channel region.

Salmon origin in California's Sacramento–San Joaquin river system as determined by otolith strontium isotopic composition

Geochemical methods for distinguishing salmon of different runs would improve management practices designed to mitigate for declines in salmon populations in California9s Sacramento–San Joaquin river system. Strontium isotopic measurements show a strong relationship between the 87 Sr/ 86 Sr ratio in hatchery water and the 87 Sr/ 86 Sr ratio in the otoliths (aragonitic ear bones) of juvenile chinook salmon ( Oncorhynchus tshawytscha ) raised in those waters. As a result of differences in basin geology from north to south along the western slope of the Sierra Nevada, important salmon spawning rivers within the Sacramento–San Joaquin river system have distinct 87 Sr/ 86 Sr ratios. Of the 10 rivers in this study, those in the Sacramento River drainage have lower 87 Sr/ 86 Sr ratios (0.7039–0.7063) than those in the San Joaquin River basin (0.7068–0.7092), with the exception of the American River, which has the highest 87 Sr/ 86 Sr ratios in this study (average 0.7100). The combination of distinct river 87 Sr/ 86 Sr ratios and the relationship between water and otolith Sr isotope ratios indicates that this geochemical method can be used to identify the origin (and potentially the migration history) of juvenile, out-migrating salmon in the Sacramento–San Joaquin system.

Intrashell Radiocarbon Variability in Marine Mollusks

We demonstrate variable radiocarbon content within 2 historic (AD 1936) and 2 prehistoric (about 8200 BP and 3500 BP) Mytilus californianus shells from the Santa Barbara Channel region, California, USA. Historic specimens from the mainland coast exhibit a greater range of intrashell variability (i.e. 180-240 14C yr) than archaeological specimens from Daisy Cave on San Miguel Island (i.e. 120 14C yr in both shells). δ13C and δ18O profiles are in general agreement with the upwelling of deep ocean water depleted in 14C as a determinant of local marine reservoir correction (∆R) in the San Miguel Island samples. Upwelling cycles are difficult to identify in the mainland specimens, where intrashell variations in 14C content may be a complex product of oceanic mixing and periodic seasonal inputs of 14C-depeleted terrestrial runoff. Though the mechanisms controlling ∆R at subannual to annual scales are not entirely clear, the fluctuations represent significant sources of random dating error in marine environments, particularly if a small section of shell is selected for accelerator mass spec- trometry (AMS) dating. For maximum precision and accuracy in AMS dating of marine shells, we recommend that archae- ologists, paleontologists, and 14C lab personnel average out these variations by sampling across multiple increments of growth.

Stable isotope and salinity systematics in estuarine waters and carbonates: San Francisco Bay

Salinities, δD and δ18O values of water samples collected bimonthly from two stations in San Francisco Bay estuary during 1991–1993, and along a salinity transect in March of 1992, indicate a linear mixing relation between the isotopic compositions of the waters and their salinities. The salinities and stable isotope compositions of samples from two locations in San Francisco Bay vary in response to changes in freshwater inflow. The data from these locations indicate simple mixtures of Pacific Ocean water (salinity ≈33, δ18O ≈ 0 to −1‰, δD ≈ 0 to −10‰) and Sacramento-San Joaquin River water (salinity ≈ 0, δ18O = −10 to −12‰, δD = −75 to −85‰). Preliminary water balance estimates, using isotopic differences between local and upland runoff, suggest that local runoff (including waste water) comprises less than 20% of total freshwater entering the bay. The average δ18O values of mussel shells (Mytilus edulis) collected live from eight locations in San Francisco Bay primarily reflect the δ18O of the water in which they grew. Shells subsampled along growth bands show that seasonal shifts in salinity and δ18O are recorded in the shells. Therefore, the use of stable isotope measurements should be useful in reconstructing pre-instrumental bay salinity and associated freshwater inflow (both annual average values and seasonal variations) to the San Francisco Bay, as well as potentially other estuarine systems.

Coral ages and island subsidence, Hilo drill hole

A 25.8-m-thick sedimentary section containing coral fragments occurs directly below a surface lava flow (the ∼1340 year old Panaewa lava flow) at the Hilo drill hole. Ten coral samples from this section dated by accelerator mass spectrometry (AMS) radiocarbon and five by thermal infrared multispectral scanner (TIMS) 230Th/U methods show good agreement. The calcareous unit is 9790 years old at the bottom and 1690 years old at the top and was deposited in a shallow lagoon behind an actively growing reef. This sedimentary unit is underlain by a 34-m-thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single 14C date of 10,340 years. The age-depth relations of the dated samples can be compared with proposed eustatic sea level curves after allowance for island subsidence is taken. Island subsidence averages 2.2 mm/yr for the last 47 years based on measurements from a tide gage near the drill hole or 2.5–2.6 mm/yr for the last 500,000 years based on the ages and depths of a series of drowned coral reefs offshore from west Hawaii. The age-depth measurements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores. The Panaewa lava flow entered a lagoon underlain by coral debris and covered the drill site with 30.9 m of lava of which 11 m was above sea level. This surface has now subsided to 4.2 m above sea level, but it demonstrates how a modern lava flow entering Hilo Bay would not only change the coastline but could extensively modify the offshore shelf.

Stable isotope record of late Holocene salinity and river discharge in San Francisco Bay, California

Abstract Oxygen and carbon isotopic measurements of fossil mollusks from San Francisco Bay are used to derive a record of paleosalinity and paleostreamflow for the past 5,900 years. The δ 18 O and δ 13 C values of river water (−12‰ and −9‰) are markedly different than seawater (0‰ and 1‰), and vary systematically as a function of salinity in the estuary. The data show that annually averaged salinity in the south-central part of the Bay was very close to the modern ‘diversion-corrected’ value of 26.8‰ over the past 2,700 years, and 4‰ lower than modern between 3,800 and 5,100 yr B.P. Based on those salinities, the average annual river inflow to San Francisco Bay is calculated to have been 1290 m 3 /s over the past 2,400 years, and 1990 m 3 /s between 3,800 and 5,100 yr B.P., 1.8 times greater than the modern ‘diversion-corrected’ value of 1100 m 3 /s, assuming a constant bay volume. The inferred river discharge record generally corroborates independent paleohydrologic records in California, including tree-ring, treeline and lake level records.

Strontium Isotopic Composition of Mid-Cretaceous Seawater

The 87Sr/86Sr ratio in fish teeth separated from mid-Cretaceous marl and black shale from the northeastern Apennines and Venetian Alps (Italy) define three periods of low 87Sr/86Sr ratio at 121 to 124 million years ago (Ma), 110 to 115 Ma, and 89 to 91 Ma. The 87Sr/86Sr excursions correspond to oceanic anoxic events represented by the Livello Selli, Livello 113, Livello Urbino, and Livello Bonarelli black shale marker beds and probably reflect an increase in the low-87Sr/86Sr hydrothermal strontium flux associated with the emplacement of the Ontong-Java and Kerguelen plateaus (120 to 110 Ma) and the Caribbean Plateau (89 to 91 Ma). The modeled flux is consistent with the volumes and eruption rates of the oceanic plateaus but is far smaller than expected from the proposed Cretaceous crustal production rates of 50 to 100 percent greater than modern.

Marine evidence for episodic Holocene megafloods in North America and the northern Gulf of Mexico

Hemipelagic muds deposited during the past 5.3 cal kyr in the northern Gulf of Mexico (Orca Basin) contain seven intervals punctuated by relatively coarse siliciclastic grain-size peaks, planktonic faunal turnovers, and negative δ13C excursions. We believe these episodes represent megaflood deposits reflecting historically unprecedented outfall of North American floodwater and terrigenous mud plumes into the gulf, resulting in collapse of the open-ocean pelagic ecosystem. The deposits record multidecadal episodes of high continental precipitation and large Mississippi River floods at ∼4.7, 3.5, 3.0, 2.5, 2.0, 1.2, and 0.3 cal ka (500–1200-year recurrence interval). Variations in tropical plankton frequencies define submillenial warming intervals that culminate in these fluvial episodes. Strengthened tropical currents in the gulf at these times appear to have increased sea surface temperatures and associated flow of moist gulf air to the midwest. Terrestrial paleohydrologic records support the marine evidence for millennial-scale changes in recurrence of large midwest flood episodes.