F.G. Prahl*, G.V. Wolfe† and M.A. Sparrow*
*College of Oceanic and Atmospheric Sciences,
Oregon State University, Corvallis, OR 97331-5503
†Department of Biological Sciences,
California State University, Chico, CA 95929-0515
ABSTRACT
We conducted isothermal (15oC) batch culture experiments with the coccolithophorid
Emiliania huxleyi (strain NEPCC 55a) to evaluate the extent to which
nutrient and light stress contribute to variability in the alkenone unsaturation
index UK’37. Alkenone content and composition were constant throughout
exponential growth in both experiments when nutrients (nitrate, o-phosphate)
were replete. Stationary phase (nutrient-starved) cells continued to produce
alkenones, amassing concentrations (SAlk) up to 3x
higher than those dividing exponentially (1.5-2 pg/cell), and the UK'37 of ‘excess’
alkenone dropped by 0.11 units. In contrast, five days of continuous darkness
resulted in a 75% decrease in cellular SAlk
and a significant UK'37 increase (+0.11 units). Given an established 0.034 unit/oC
response for exponentially growing cells of this strain, the observed range
of UK’37 variability at 15oC corresponds to an uncertainty of +/-3.2oC
in predicted growth temperature. This level of variability matches that in the
global UK'37 – annual mean sea-surface temperature calibration for surface
marine sediments, begging the question: what is the physiological condition
of alkenone-producing cells exported to marine sediments? Comparison of our
laboratory results for a strain of E. huxleyi isolated from the subarctic Pacific
Ocean with depth profiles for alkenones in surface waters from two contrasting
sites in the NE Pacific Ocean suggests the answer to this question depends on
the ocean regime considered, a possibility with significant bearing on how stratigraphic
UK'37 records in marine sediments are to be interpreted paleoceanographically.
Selective Organic Matter Preservation
in ‘Burndown’ Turbidites on the Madeira Abyssal Plain (2003)
F.G. Prahl†¥, G.L. Cowie£, G.J. De Lange#, M.A. Sparrow†
†College of Oceanic and Atmospheric Sciences, Oregon State University,
Corvallis, OR 97331-5503, USA
£Marine Geosciences Unit, Geology and Geophysics Department, University
of Edinburgh, West Mains Road, EH9 3JW, Scotland
#Institute of Earth Sciences, Department of Geochemistry, Utrecht University,
3584 CD, Utrecht, The Netherlands
ABSTRACT
Oxidized and unoxidized intervals of five distal, organic-rich turbidites deposited
on the Madeira Abyssal Plain (MAP) during three different geological time periods
(Miocene, Pliocene, Pleistocene) were analyzed for a common set of bulk geochemical
and molecular-level lipid biomarker properties. In every case, aerobically oxidized
intervals displayed major loss of total organic carbon (TOC, 84±3.1%)
accompanied by a negative shift in carbon isotopic composition (d13C)
ranging from -0.3 to -2.9‰. Major, but significantly lower, loss of total
nitrogen (Ntot, 61±7.1%) also occurred, leading to a conspicuous decrease
in the relative TOC to Ntot content (C/Ntot) of each deposit and a +1.3 to +2.7‰
shift in the isotopic composition of Ntot (d15N).
Two possible causes could account for the negative d13C
shift: 1) selective preservation of 13C-depleted terrestrial relative to 13C-enriched
marine organic matter or 2) selective preservation of a 13C-depleted component
of marine organic matter. Compound specific d13C
measurements on long-chain alkenones of marine origin and plantwax n-alkanes
of terrestrial origin provide support favoring the first alternative. The decrease
in C/Ntot and positive shift in d15N upon oxidation of the turbidite seems at
face value inconsistent with selective preservation of terrestrial organic matter.
However, results from organic geochemical analysis of a Modern eolian dust sample
collected in the vicinity of MAP indicate that the observations for C/Ntot and
d15N are not incongruous. Regardless of how the isotopic
shifts are specifically explained, collective results from study of the aerobic
‘burndown’ phenomena that acted on MAP turbidites highlight the
message that reconstruction of paleoenvironmental properties such as pCO2 using
the d13C record of TOC in sediments is a research
goal fraught with uncertainty whether or not the marine record under consideration
is ‘contaminated’ with significant quantities of terrestrial material.
Nonetheless, despite major and selective loss of both marine and terrestrial
components as a consequence of postdepositional aerobic oxidation, at least
some proxies based on specific molecular constituents of the TOC such as the
alkenone unsaturation index, UK’37, survive quite unimpaired and retain
their promise for use in paleoceanographic study.
A particle conveyerbelt process in the Columbia River-Estuary: evidence from chlorophyll a and particulate organic carbon. 2004. Estuaries 27(6):999-1013.
LAWRENCE F. SMALL AND FREDRICK G. PRAHL
Abstract -- Using both the photosynthetically active chlorophyll a (chl a) content of the organic carbon fraction of suspended particulate matter (chl a/POC) and the percentage of photosynthetically active chl a in fluorometrically measured chl a plus pheophytin a (% chl a), we determined that under specified hydrodynamic conditions, neap-spring tidal differentiation in particle dynamics could be observed in the Columbia River estuary. During summertime neap tides, when river discharge was moderate, bottom chl a/POC remained relatively unchanged from riverine chl a/POC over the full 0–30 psu salinity range, suggesting a benign trapping environment. During summertime spring tides, bottom chl a/POC decreased at mid range salinities, indicating resuspension of chl a-poor POC during flood-ebb transitions. Bottom % chl a during neap tides tended to average higher than that during spring tides, suggesting that neap particles were more recently hydrodynamically trapped than those on the spring tides. Such differentiation supported the possibility of operation of a particle conveyor belt process, a process in which low-amplitude neap tides favor selective particle trapping in estuarine turbidity maxima (ETM), while high-amplitude spring tides favor particle resuspension from the ETM. Untrapped river-derived particles at the surface would continue through the estuary to the coastal ocean on the neap tide; during spring tide some particles eroded from the ETM would combine with unsettled riverine particles in transit toward the ocean. Because intensified biogeochemical activity is associated with ETM, these neap-spring differences may be critical to maintenance and renewal of populations and processes in the estuary. Very high river discharge (15,000 m3 s-1) tended to overwhelm neap-spring differences, and significant oceanic input during very low river discharge (5,000 m3 s-1) tended to do the same in the estuarine channel most exposed to ocean input. During heavy springtime phytoplankton blooms, development of a thick bottom fluff layer rich in chl a also appeared to negate neapspring differentiation because spring tides apparently acted to resuspend the same rich bottom material that was laid down during neap tides. When photosynthetic assimilation numbers [mgC (mgchl a)-1 h-1] were measured across the full salinity range, no neap-spring differences and no river discharge effects occurred, indicating that within our suite of measurements the compositional distinction of suspended particulate material was mainly a function of chl a/POC, and to a lesser extent % chl a. Even though these measurements suggest the existence of a conveyor belt process, proof of actual operation of this phenomenon requires scalar flux measurements of chl a properties in and out of the ETM on both neap and spring tides.