Paleo Sea Surface Temperature Research Articles

Climatic and oceanographic variations on the California continental margin during the last 160 kyr

Organic matter in sediment samples from three ODP sites (Ocean Drilling Program Leg 167) that form a south-north transect was investigated to reconstruct the paleoclimatic and oceanographic conditions on the California continental margin during the last 160 kyr. Alkenone-derived paleosea surface temperatures (SST) are 3 to 6°C colder in glacial stages and reveal a clear relationship with global climate changes; the differences are greater in the north. Latitudinal SST comparison exhibits water mixing of the colder California Current with warmer waters from the south, particularly in the southern central California borderland area. Organic matter accumulation on the California continental margin indicates an interplay between climatic and atmospheric glacial–interglacial variations and spatially and temporally changing nutrient availability along the California coastline. Climatic and atmospheric dependent circulations apparently caused variations in the intensity of coastal upwelling along the southern central California margin and this suggests, due to the close connection of the California Current to the local wind patterns, that the California Current was weaker during glacial and stronger during interglacial periods.

Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction

The successful reconstruction of sea surface temperatures using alkenone paleothermometry (U 37 k′) has relied on the premise that there is no significant differential degradation of alkenones with different states of unsaturation during diagenetic processes. To test this assumption, we conducted a comparative study of contemporary sediments in oxic and anoxic bottom waters from the Santa Monica Basin, offshore California. Long-chain alkenones were quantified and sea surface temperature were calculated using the calibrated U 37 k′–T relationship of Prahl et al. (1988). Our results show that temperature record from the oxic sediments is higher by as much as 4°C compared to those from time-equivalent anoxic sediments as a result of differential degradation of long-chain unsaturated alkenones and bioturbation mixing in the oxic sediments. The differential degradation of C 37:3 vs. C 37:2 alone could account for up to 2.5°C difference between these two records. This finding has significant implication in the interpretation of paleo–sea surface temperature data using alkenone paleothermometry.

Genetic and Physiological Influences on the Alkenone/Alkenoate Versus Growth Temperature Relationship in Emiliania huxleyi and Gephyrocapsa Oceanica

Selected warm and cold water strains of the coccolithophorid Emiliania huxleyi and the closely related species Gephyrocapsa oceanica were cultured under controlled temperature conditions to assess genetic and physiological variability in the alkenone/alkenoate vs. temperature relationship. Differences in the strains’ growth rates over the 6–30°C experimental temperature range were small but consistent with their cold or warm water origins. E. huxleyi and G. oceanica had similar alkenone/alkenoate biochemistry, justifying the extension of alkenone stratigraphy to sediments predating the appearance of E. huxleyi. These species could not be distinguished by C 38/C 37 alkenone or alkenoate/alkenone ratios as previously suggested ( Volkman et al. 1995; Sawada et al. 1996) but given samples from a range of temperatures may be distinguished by a plot of the C 38 ethyl vs. C 38 methyl unsaturation ratios (U 38Et K and U 38Me K, respectively). Biochemical responses to temperature and the C 37 alkenone-based (U 37 K′) temperature calibrations differed significantly among the strains. The U 37 K′ temperature calibration was nonlinear for five of the six strains examined. A reduction in slope of the calibration at temperatures < 12°C and >21°C suggests the cell’s alkenone-based adaptation to temperature is limited at the extremes of its growth temperature range. The unsaturation ratios of the C 38 methyl and ethyl alkenones (U 38Me K and U 38Et K) varied similarly with temperature and were strongly intercorrelated. The experiments also documented an influence of cell physiological state on both alkenone and alkenoate composition and on alkenone unsaturation. Cells in late logarithmic and stationary growth had significantly increased abundance of alkenoates and C 38 ethyl alkenones relative to C 38 methyl alkenone abundance. In some strains the unsaturation ratios of both C 37 and C 38 alkenones also significantly decreased when cells entered the late log phase. Comparison of culture results with field data indicates that the average physiological state of alkenone-synthesizers in the open ocean differs from cultured cells growing under exponential growth and appears to be more similar to cells in late log or stationary growth phases. Differences in alkenone/alkenoate ratios between cultured cells and sediments underlying waters of a similar temperature most probably reflect a difference in cell physiology between cultured cells and oceanic populations and not greater diagenetic losses of alkenones relative to alkenoates, as previously suggested ( Prahl et al. 1995). Our experiments confirm that biogeographical variations observed in the alkenone vs. temperature relationship in natural waters reflect, at least in part, differences in genetic makeup and physiological status of the local alkenone-synthesizing populations. Hence, alkenone-based paleo sea surface temperature estimates are subject to errors, albeit small, which arise from genetic differences between modern-day and paleo-populations. The reduction in slope of the U 37 K′ temperature calibration for most strains at T >24°C indicate that linear U 37 K′ temperature calibrations (e.g., Prahl et al. 1988) which are currently used to estimate paleo SST, and which are poorly constrained at higher temperatures, probably underestimate the magnitude of SST change for tropical and subtropical regions.

Lipídios como Indicadores de Mudanças Climáticas no Passado 1: Biomarcadores Marinhos

The distribution and abundance of specific lipid biomarkers in marine sediments can play an important rule for the reconstruction of past climatic changes. Here, it is shown the use of a widely known paleo Sea Surface Temperature (SST) proxy based on the unsaturation ratio of a suite of long-chain C37 alkenones: the alkenone unsaturation index, UK37'. These compounds are biosynthesised by only a restricted group of prymnesiophyte algae, mostly notably the coccolithophorid Emiliania huxleyi. To ilustrate the importance of the UK37' index as a molecular tool, the correlation between the SST signal derived from the UK37' and other paleoclimatic records examined in a core from the North Atlantic is discussed.

Alkenone sea surface temperature in the Southern Ocean for the last two deglaciations

A piston core taken from the Tasman Plateau in the Southern Ocean has been studied for organic compounds to reconstruct the late Pleistocene marine environments. Here we report paleo sea surface temperature (SST) for the last two deglaciations using the unsaturation degree of alkenones (Uk37′) preserved in the marine sediments. The Uk37′ record indicates that the SST was at least 4°C lower than the present SST at the last glacial maximum (LGM), and the amplitude of paleo‐SST is at most 5.2°C from the penultimate glacial (MIS‐6) to the last interglacial warm period (the Eemian). Our results also demonstrate that the Eemian warm period in the Southern Ocean lasted only 3,000 years followed by a sharp cooling at around 120 kyr BP. The sharp cooling in the Southern Ocean seemed to occur a few millennia (2‐3 kyrs) before the beginning of continental ice‐sheet growth.

Late Cretaceous Oceans and the Cool Tropic Paradox

Oxygen isotopic proxies of paleo-sea surface temperatures (SSTs) suggest that Maastrichtian (about 66 million years ago) tropical SSTs were lower than those of today. They also demonstrate that Maastrichtian latitudinal SST gradients were much lower than those of the present. The low Maastrichtian SST gradients indicate that meridional heat transport was much greater or latitudinal differences in the balance of radiation to and from the sea surface were much less extreme during the latest Cretaceous than they are today, or that both conditions were true. These findings challenge traditional interpretations of “greenhouse” Late Cretaceous climates.

Geochemical and sedimentologic variations in inter-annually laminated sediments from Santa Monica Basin

Laminated sediments were recently discovered in the center of Santa Monica Basin, California Continental Borderland and extend from the present day to at least the Little Ice Age (ca. 1600 A.D.). Unlike the annual varves observed in Santa Barbara Basin, the light-dark lamination couplets observed in 210Pb-dated cores from the center of Santa Monica Basin occur at 3 to 6 year intervals, very similar in length to the El Niño Southern Oscillation (ENSO) phenomena. The mechanisms that control the temporal patterns of laminations in this basin are unknown. Potential causal mechanisms of lamination formation in Santa Monica Basin include: (1) changes in the flux of terrigenous vs. marine organic carbon and detritus; (2) surface water processes which affect marine productivity and organic carbon flux to the sediments; and (3) deep water processes which affect oxygen renewal and preservation of organic matter within the basin. A box core from the central portion of Santa Monica Basin was analyzed using a suite of geochemical, sedimentary, and micropaleontologic techniques in order to: (a) investigate which, if any, of these mechanisms has contributed to the occurrence of laminations; and (b) reconstruct proxy records of climate change. Samples were extracted from this core at 1 mm increments and provide a temporal resolution of 1–3 years per sample, a resolution which is comparable in length to the thickness of individual laminae within the core. Geochemical and micropaleontologic analyses of core samples provide paleoclimatic and paleoceanographic records which are comparable to, and extend beyond, historical regional and global climate records. In addition to direct comparisons between these records, spectral analysis was used to investigate whether there were similar patterns of variability that would implicate dominant forcing functions in the historic and proxy records. Herein we present preliminary results of records from ca. 1860 to the present. Our results suggest that organic carbon fluxes are the dominant factors that affect lamination variability. Input of organic carbon is primarily a function of primary productivity, and, to a lesser extent, a function of percentage of terrestrial input. Thus, (1) and (2) are both mechanisms of lamination formation. Stability of (3) only allows preservation of the record of these variations. Variations in Southern California rainfall appear to be linked to decadal-scale variations in downcore density profiles, and thus cause variations in the thickness and character of the laminations in the deepest portions of this basin. Thus, filtered density records can be used to reconstruct general paleo-rainfall trends which compare well with historic rainfall records. Variations in the isotopic composition of total organic carbon (TOC) are compared to historic ENSO and climate records and primarily reflect changes in the relative contribution of marine organic carbon to the basin. Variations in the carbon isotopic composition of N. pachyderma mirror changes in the sedimentary δ 13C TOC record from Santa Monica Basin, and suggest that primary productivity is the primary control on the flux of organic carbon to the sediments and exerts a strong influence on the interannual variability in lamination composition. Variations in paleo-sea surface temperatures generated from the oxygen isotopic composition of N. pachyderma correlate well with historical SST records and suggest potential usage as a proxy for general paleoSST patterns. Oxygen-sensitive assemblages of benthic foraminifera ( Bolivina argentea and B. spissa) have been used to identify potential large-scale flushing events and construct a historical record of variations in deep water oxygen levels. Spectral analysis of regional historic records indicate dominance of short-term (ENSO-length) and long-term (decadal) patterns of climate variability. The short-term ENSO signal is not dominant in the Santa Monica proxy records. However, strong decadal-length signals are identified in all of the proxy records and suggest that the dominant forcing mechanisms in Santa Monica Basin may be decadal in length.