Catherine A. Rigsby

Tropical Climate Changes at Millennial and Orbital Timescales on the Bolivian Altiplano

Tropical South America is one of the three main centres of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the ‘Walker’ circulation). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximum—marked in the drill core by continuous deposition of lacustrine sediments—appear to have occurred in phase with summer insolation maxima produced by the Earths precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as well as the Younger Dryas episode. At both millennial and orbital timescales, cold sea surface temperatures in the high-latitude North Atlantic were coeval with wet conditions in tropical South America, suggesting a common forcing.

The deep structure of a sea-floor hydrothermal deposit

Hydrothermal circulation at the crests of mid-ocean ridges plays an important role in transferring heat from the interior of the Earth. A consequence of this hydrothermal circulation is the formation of metallic ore bodies known as volcanic-associated massive sulphide deposits. Such deposits, preserved on land, were important sources of copper for ancient civilizations and continue to provide a significant source of base metals (for example, copper and zinc). Here we present results from Ocean Drilling Program Leg 169, which drilled through a massive sulphide deposit on the northern Juan de Fuca spreading centre and penetrated the hydrothermal feeder zone through which the metal-rich fluids reached the sea floor. We found that the style of feeder-zone mineralization changes with depth in response to changes in the pore pressure of the hydrothermal fluids and discovered a stratified zone of high-grade copper-rich replacement mineralization below the massive sulphide deposit. This copper-rich zone represents a type of mineralization not previously observed below sea-floor deposits, and may provide new targets for land-based mineral exploration.

Holocene multidecadal- to millennial-scale hydrologic variability on the South American Altiplano

Precipitation on the South American Altiplano varies at a range of temporal scales. A long-term secular increase in moisture availability from the early/mid Holocene to the present, driven by increasing summer insolation resulting from precessional changes in the Earths orbit, has been documented in earlier studies. However, higher frequency Holocene variability is not yet understood. Here we present high-resolution diatom assemblage data from two small Altiplano lakes, Lago Lagunillas and Lago Umayo, indicating changes in effective moisture in the southern tropical Andes at decadal, centennial and millennial timescales throughout the mid to late Holocene. A strong millennial-scale component, similar in pacing to periods of increased icerafted debris flux in the North Atlantic, is observed in both lake records, which suggests that regional precipitation and North Atlantic climate variability are coupled at these scales. The interpretation of the higher frequency variability is hampered by the small number of high-resolution continental and marine records for comparison.

Application of strontium isotopes to understanding the hydrology and paleohydrology of the Altiplano, Bolivia–Peru

Abstract Strontium concentrations and strontium isotopic ratios were measured in natural waters and carbonate sediments from throughout the Bolivian and Peruvian Altiplano in order to improve hydrologic and paleohydrologic mass balances with the ultimate goal of better understanding the paleoclimatic history of the central Andes. Rivers flowing into Lake Titicaca have a wide range of strontium isotopic ratios that exhibit spatial patterns consistent with the lithologies of the different drainage basins. Because of the limited exchange of water between the two main sub-basins of Lake Titicaca, Lago Grande and Lago Huinaimarca, and between the sub-basins of Lago Huinaimarca, there are significant differences in strontium isotopic ratios between the sub-basins. Calculated elemental balances of strontium in Lake Titicaca are in reasonable agreement with previously published budgets of water and major elements. However, the strontium isotopic budget indicates that the lake is not in isotopic steady state. This also implies that the major-element budgets are unlikely to be in steady state. Lake Titicaca had a higher-than-modern strontium isotopic ratio during the early and middle Holocene. Elevated values persisted in Lago Grande until at least 2000 cal yr BP, consistent with other evidence that modern hydrologic conditions (namely overflow) were not established until that time. An isotopic budget calculated for late-Pleistocene paleolake Tauca in the central Altiplano suggests that between 70% and 83% of its riverine inputs were derived from Lake Titicaca overflow. This calculated flow represents about a 30-fold increase over the average discharge of the modern Rio Desaguadero at Desaguadero or a seven-fold increase over its average discharge into Lago Poopo. The strontium isotopic budget (if complete) precludes the possibility that decreased evaporation alone could have accounted for the existence of paleolake Tauca.

Fluvial history of the Rio Ilave valley, Peru, and its relationship to climate and human history

Fluvial strata and landforms in the Rio Ilave valley (Peru) document a history of Holocene aggradation and downcutting that is correlative with regional climatic events and provides an environmental context for human occupation of the river valley. Periods of aggradation correspond to periods of high (or rising) level in Lake Titicaca and elsewhere on the Altiplano, and increased sediment accumulation in the Rio Ilave valley. Downcutting episodes correspond to periods of low level in Lake Titicaca and low or rapidly decreasing sedimentation rates in the Ilave delta. There are five terrace tracts (T1 through T5) present in this southwestern Lake Titicaca tributary. These tracts occur as both paired and unpaired terraces and have average heights from 1.4 to 24.3 m above the valley floor. The major part of the fluvial sequence was deposited during the time period from prior to the Last Glacial Maximum until about 8300 calendar years Before Present (cal BP) ^ a period of generally high (but variable) precipitation on the Altiplano and high water level in Lake Titicaca. Initial deposition (aggradation) was followed by successive downcutting to the T4 and T3 terrace surfaces. Initial downcutting began immediately after precipitation, runoff, and sediment load decreased while base level dropped. It was followed by a period of episodic equilibrium and minor downcutting that included a prolonged period of soil formation between V8350 and 6780 cal BP. The major pulses of downcutting likely occurred between V6000 and 4500 cal BP and were coincident with periods of decreased precipitation on the Altiplano and decreasing levels of Lake Titicaca. Two final periods of infilling, resulting in deposition of the T2 and T1 terrace sediments at V4000 to 2500 cal BP and V2000 to 1600 cal BP (during periods of rising water level in Lake Titicaca, lacustrine sedimentation in the Rio Desaguadero valley, and increased sedimentation offshore the Ilave delta), were separated by brief equilibrium stages and a brief downcutting event. This fluvial history, when coupled with regional paleoclimatic data, relates to the region’s preceramic through Tiwanakuperiod archeological records. Archeological evidence indicates that humans occupied the Ilave valley as early as 10 000 cal BP. The higher terraces (T3, T4 and T5) were occupied for at least 5000 years, but humans did not utilize the lower terraces (T1 and T2) until after V4400^3700 cal BP. Our results confirm that these lower terraces would not have been available for either occupation or agriculture until after V4000 cal BP. ; 2003 Published by Elsevier Science B.V.

The Nature and Origin of Decadal to Millennial Scale Climate Variability in the Southern Tropics of South America: The Holocene Record of Lago Umayo, Peru

This paper serves two purposes: to review current ideas about the nature and forcing of decadal to millennial scale precipitation variation in the southern tropics of South America during the late Quaternary and to present a new methodology for the reconstruction of precipitation as applied to a Holocene stable isotopic record of carbonate sediments in a tropical Andean lake, Lago Umayo, Peru. The basic thesis of the first part of the paper is that, although modern instrumental records suffice for deducing climate variability at decadal and shorter time scales, these records cannot adequately characterize the nature and forcing of lower-frequency climate variation. Understanding the nature of multi-decadal to millennial-scale climate variation and the mechanisms of large abrupt climate change is best derived from paleoclimatic time series. Tropical Atlantic sea-surface temperature variation is a significant control on tropical South American paleoclimate at these longer time scales. In the second part of the paper, an original method is presented for quantitatively reconstructing precipitation. This method utilizes the well-known relationship between the stable isotopic composition of precipitation and the amount of precipitation, a relationship that is highly significant in many tropical locales. Due to many simplifying assumptions, the reconstruction should be considered to be tentative.

Floodplain Sedimentation During an Extreme Flood: the 1999 Flood on the Tar River, Eastern North Carolina

This study examines floodplain sedimentation following the largest flood in the 98-yr. record on the Tar River, North Carolina. Hurricane Floyd made landfall just 10 days after Hurricane Dennis in September 1999, bringing unprecedented rainfall (30-46 cm) and flooding to eastern North Carolina. A field survey of the lower 350 km of the river showed that this >500 yr. flood deposited very little overbank sediment (<1 mm) on most of the floodplain. We used suspended sediment concentrations measured on the Tar River from 1958-1967 to suggest that the seasonal timing and sequencing of flood events in 1999 are the most probable explanations for the minimal geomorphic impact of this extreme flood. The early autumn timing of the flood coincided with crops that were mature but not yet harvested, and when natural vegetation was very dense and effective at stabilizing channel banks, hillslopes, and floodplain soils. Hurricane Dennis may have exhausted the available sediment supply and transported this sediment to the Pamlico Sound before reaching flood stage, thereby reducing the sediment available to be transported and deposited by the flood that followed Hurricane Floyd.

Deepening-upward Sequences in Oligocene and Lower Miocene Fan-delta Deposits, Western Santa Ynez Mountains, California

ABSTRACT Coarse, matrix-supported conglomerates, an abundant invertebrate faunal assemblage, and large vertical burrows record high-energy nearshore deposition in the Vaqueros Formation of the westernmost Santa Ynez Mountains. These strata document the effect of Late Oligocene-Early Miocene eustatic sea-level rise on fan-delta deposits of the tectonically active southern California continental margin. The Vaqueros fan-delta succession can be divided into six facies deposited in three fan-delta facies assemblages: fan-delta plain, fan-delta front, and fan-delta slope. The fan-delta-plain facies assemblage consists of inversely to normally graded, matrix-supported, boulder to pebble conglomerates. These conglomerates are angular, nonfossiliferous, and laterally persistent. The fan-delta front facies assemblage is characterized by non-graded to normally graded, sandy, cobble to pebble conglomerates and conglomeratic sandstones organized in both channelized and nonchannelized bedding units. Pectens, oysters, and shallow-water gastropods are abundant in this facies, and individual beds have sharp bases and burrowed tops. Small-scale lenticular bedding and reworked lenses of fossil mater al are common throughout. Locally, the nonchannelized units show subhorizontal bedding. Fan-delta-slope strata are dominated by extensively bioturbated, locally cross-bedded, pebbly, coarse sandstone overlain by sandstone/fossil conglomerate couplets. Pebble imbrication throughout the sequence, along with channel orientations and cross-bedding in the fan-delta-front units, record paleoflows to the east and the southeast. These facies assemblages represent deposition in a deepening-upward fan-delta system on the southern flank of the then-emergent Santa Maria basin (the paleotrench-slope break). Fan-delta-plain conglomerates record braided fluvial and intertidal deposition; fan-delta-front conglomerates and sandstones record channel-mouth and bar/spit deposition; and the fan-delta-slope sandstones record deposition in shoreface to inner-shelf environments. The sequence documents local sedimentary response during a period of regional tectonic subsidence and general eustatic sea-level rise.