On the occurrence of long-chain n-alkyl diols and hydroxy ketones
in Holocenic sediments from an upwelling region in SW Atlantic
ocean (Cabo Frio, Brazil)
Aída P. Baeta1, Renato S. Carreira2,*, Angela L.R. Wagener1, Luiz C. Silva1,
Ana L. S. Albuquerque3, Abdelfettah Sifeddine4
1 LABMAM/Departamento
de Química, Pontifícia Universidade Católica do
Rio de Janeiro, 22453-900 Rio de Janeiro, Brazil
2
LaGOM/Faculdade de Oceanografia, Universidade do Estado do Rio de
Janeiro, 20550-013 Rio de Janeiro, Brazil
3 Departamento
de Geoquímica, Universidade Federal Fluminense, 24020-
007 Niteroi, RJ, Brazil.
4Institute
de Recherche pour le Developpement, Bondy, France
*corresponding author: Tel.: +55 21 2587-7689; fax: +55 21 2587-7692;
email: carreira@uerj.br
Abstract
The applicability of diols and keto-ols as markers in paleoenvironmental
reconstructions was evaluated considering sediments accumulated from 0.7 to
13.0 kyears BP (14C age) in a tropical upwelling region in the SW Atlantic
(Cabo Frio, Rio de Janeiro). The flux of organic carbon (0.27 ± 0.13 and 1.26 ±
0.36 mg C cm-2 year-1 ) and the mean values from 80.6 to 86.1 for the
calculated diol index (Versteegh et al., 1997) were consistent with occurrence
of relatively weak and intermittent upwelling events. However, the association
of lower diol index values (<60 to 79) with higher rates of organic carbon
accumulation indicates enhanced upwelling conditions during events of higher
primary production. Keto-ols appeared in very low concentrations in all
samples. The results show that diol’s occurrence and distribution in the
studied sediments is a valuable tool to be used in association other lipid
biomarkers to investigate climate dependent upwelling conditions in Cabo
Frio.
Key-words: Upwelling, Cabo Frio, paleoproductivity, sediment, Holocene,
diol, keto-ol
1. Introduction
Long-chain n-alkyl diols and hydroxyl ketones, or diols and keto-ols in
the abbreviated form, are structurally related compounds which are ubiquitous
and usually abundant in the lipid fraction obtained from Quaternary
sediments collected in distinct oceanic regions (see the review in Volkman,
2006). A long stratigraphic range, occurrence in a wide variety of sediments
and typical distribution within the marine realm are characteristics of these
compounds which allow their use in paleoenvironmental studies, especially in
zones of high primary productivity (Versteegh et al., 2000; Zhao et al., 2006).
In the south-eastern Brazilian continental margin, wind-driven coastal
upwelling of South Atlantic Central Water (SACW) occurs in spring and
summer, being especially intense in the region of Cabo Frio (23ºS and 43ºW)
(Gonzalez-Rodriguez et al., 1992). The upwelling of SACW may also occur in
the shelf break due to interactions of meanders and cyclonic eddies in the
Brazil Current with topographic features (Castelao & Barth, 2006 and
references therein). Although less significant than in the eastern-Atlantic, the
upwelling in Cabo Frio has great influence over the pelagic and benthonic
biological production in regional scale (De Leo & Pires-Vanin, 2006; McManus
et al., 2007). Climatic variability during the Holocene, like the oscillation of
the South Atlantic Convergence Zone (SACZ), may have influenced the
position and intensity of the upwelling in Cabo Frio, as well as the quantity
and quality of the continental inputs. To test this hypothesis, bulk organic
carbon properties and a suite of lipid biomarkers were determined in two
sediment cores collected in the continental margin off Cabo Frio (Figure 1). In
the present work we will discuss the occurrence of diols and keto-ols in
sediments deposited between 0.7 and 13.0 kyears in association with organic
carbon
accumulation
rates
to
evaluating
their
potential
use
in
paleoenvironmental reconstructions in a tropical upwelling region.
2. Materials and Methods
Two 300-cm long sediment cores (CF02-01 and CF02-02; Fig. 1) were
collected using a Kullember-type gravity corer in August 2002 aboard the R.V.
Astro-Garoupa. Once in the laboratory the cores were subdivided in 1 cm
thickness slabs. Freeze-dried sediment samples were Soxhlet-extracted with
methanol:dichloromethane (9:1, v/v) and the neutral lipids (containing the
diols and keto-ols) were isolated after saponification of the bulk extract.
Analysis of diols and keto-ols (as trimethylsilyl-ether derivatives) were
performed by GC/MS (ThermoFinnigan Focus/DSQ) in full scan mode (50 –
850 amu). The response of characteristic fragments in the mass spectra were
used to determine the relative proportions of different co-eluting isomers with
mid-chain functionality in the positions 1,13, 1,14, 1,15 and 1,16 for C28 to C32
diols and keto-ols – details are provided by Versteegh et al. (2000).
The sedimentary geochronology was based on AMS-14C data from 7
samples for core CF02-01 and 16 samples for core CF02-02 and the CALIB
5.0.1 software (Stuiver et al., 2005). A mean reservoir effect of 430 year for the
SW Atlantic Ocean was applied to calculate radiocarbon ages. Total organic
carbon contents were determined in HCl-treated samples using a drycombustion method. The flux of total organic carbon to the sediments was
calculated considering the sediment density and porosity as well as the
geochronology model. Detailed information about the geochronology model can
be provided upon request.
3. Results and Discussion
The geochronology model gave sedimentation rates varying from 0.05 to
0.12 cm year-1 for CF02-01 and from 0.01 to 0.04 cm year-1 for CF02-02. This
shows distinct periods were covered by each core: 0.7 to 4.3 kyear BP in CF0202 and 1.0 to 13.0 kyears BP in CF02-02. More recent layers were lost in both
cases during sampling. Since the cores were sampled from sites only 10 km
apart, the significant lower sedimentation rates obtained for core CF02-02
(retrieved from a deeper site in the slope as compared to CF02-01; 124 m water
depth) reflect a complex sedimentation pattern in the upper slope of Cabo Frio
region, as observed in other regions of the south-eastern Brazilian continental
margin (Mahiques et al., 2007).
The flux of organic carbon also showed variation between sites (1.26 ±
0.36 mg C cm-2 year-1 in CF02-01 and 0.27 ± 0.13 mg C cm-2 year-1 in CF02-02;
Figure 2), which were principally determined by local sedimentation rates,
since organic carbon content was similar in both cores (data not shown).
Despite the relatively low organic carbon accumulation rates compared to
other upwelling regions (for example, Böning et al., 2005), a steady increase in
the flux is evident in core CF02-01 in the period between about 4.4 and 2.3 to
kyears BP, with peak values occurring between 2.0 and 2.5 mg C cm-2 year-1
(Figure 2-a). The same feature appears in core CF02-02, but with significant
lower peak value (max of 0.5 mg C cm-2 year-1).
Regarding the individual diols distribution, C30 diol was the most
abundant compound, followed by the C32 homolog (data not shown). For both
compounds there was a clear predominance of the 1,15 isomer over the 1,12,
1,13 and 1,14 isomers. The keto-ols showed a distribution similar to the diols
in relation to chain length and position of hydroxyl group, but their low
abundance precluded further considerations about this group of compounds.
The down-core variation of the diol index, calculated as proposed by
Versteegh et al. (1997) is shown in Figure 2. For core CF02-01 (Fig. 2-a), the
mean diol index of 86.1 ± 5.5 is comparable to values reported for open seas
and/or regions with relatively weak upwelling events (Versteegh et al., 1997,
2000). The diol index range as well as the absence of C28 1,14-diol produced by
diatoms requiring nutrient-rich waters (Proboscia sp.) (Damsté et al., 2003),
are consistent with the lower upwelling intensity in Cabo Frio. The slightly
lower mean diol index in CF02-02 (80.6 ± 8.5) may probably result from the
reduced abundance of diols in this core.
The periods around 1.8 and 2.3 kyear BP as recorded in core CF02-01
(Figure 2-a) seem to have been remarkably different in respect to the
upwelling intensity. Such feature appears as a significant shift in the diol
index that reaches levels typical of highly productivity regions (i.e., <60 – 79;
Versteegh et al., 1997). Only the minimum diol index at about 2.3 kyears is
matched by an evident peak in carbon accumulation rate. The general trends
in organic carbon accumulation rates does not find parallel in the diol index.
Although fewer data are available for core CF02-02, in several periods the diol
index is within the range of higher production, inclusively around the 2.3
kyears(Figure 2-b). These results are consistent with the observation of drier
climatic conditions associated with enhanced upwelling conditions in the
period of 1.8 to 2.4 kyears BP based on the abundance of Navicula
duerrenbergiana (pinnate diatom) in sediments from a coastal lagoon in the
same region (Laslandes et al., 2006).
4. Conclusions
Variations in the intensity of upwelling events during the Holocene in
the south-eastern Brazilian continental margin are suggested by the
occurrence and distribution of diols in the two sediment cores from Cabo Frio
region. Determination of additional proxies which is under way will reveal how
such variations relate to regional or larger scale climatic events.
Acknowledgements
The authors are grateful to the Brazilian Research Council (CNPq) for
the financial support and for a doctorate fellowship (A.P.Baeta).
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Figure 1. General view of the coast of Rio de Janeiro state and the two sampling
sites in the upwelling region of Cabo Frio (see text for details).
Figure 2. Vertical variation in the organic carbon flux (mg cm-2 year-1) and diol index in cores collected in Cabo Frio region (SE
Brazilian continental margin). In (a) values for core CF02-01 (115 m water depth) and in (b) for core CF02-02 (124 m water depth).