Rangos de ascenso marino relativo en el sur del Golfo de Vizcaya durante el Antropoceno frente al Holocen

Alejandro Cearreta

Eduardo Leorri

CiteSeerX

127
GEOGACETA, 46, 2009
Copyright ® 2009  Sociedad Geológica de España / www.geogaceta.com
Anthropocene versus Holocene relative sea-level rise rates in the
southern Bay of Biscay
Rangos de ascenso marino relativo en el sur del Golfo de Vizcaya durante el Antropoceno frente al Holoceno
Eduardo Leorri (1, 2) y Alejandro Cearreta (3)
(1) Universidade de Lisboa, Faculdade de Ciências, Centro de Geologia, Campo Grande, 1749-016, Lisboa, Portugal.
eduleorri@yahoo.es
(2) Sociedad de Ciencias Aranzadi, Calle Zorroagagaina, 11, 20014 Donostia-San Sebastián.
(3) Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/EHU, Bilbao 48080.
alejandro.cearreta@ehu.es
ABSTRACT
Con el fin de evaluar la aceleración en el ascenso relativo del nivel marino durante el Antropoceno, se
han comparado los resultados previamente establecidos para el siglo XX con los resultados obtenidos a
partir del estudio de 61 nuevos puntos indicadores del nivel marino durante el Holoceno en el sur del
Golfo de Vizcaya. Estos resultados indican un ascenso rápido hasta los circa 7000 años cal BP que se
puede estimar en 9-12 mm año-1. Desde entonces, el nivel del mar ha ascendido suavemente en
concordancia con los datos proporcionados a nivel global, con una velocidad media de 0,7 mm año-1.
Esta velocidad contrasta netamente con la tasa de ascenso registrada durante el siglo XX de 1,9 mm año-1,
confirmando la aceleración detectada a nivel global como resultado de las actividades antropogénicas.
Aunque los resultados obtenidos son altamente satisfactorios, los diferentes rangos de error proporcionados
por ambos estudios hacen necesarios más estudios de este tipo para reducir los errores asociados a estas
reconstrucciones.
Key words: Foraminíferos bentónicos, estuarios, Holoceno, nivel marino, Golfo de Vizcaya
Geogaceta, 46 (2009), 127-130
ISSN: 0213683X
Introduction
Leorri and Cearreta (2009a)
calibrated the foraminiferal assemblages
of two salt-marsh cores from two
estuaries using a regional transfer
function constructed to the southern Bay
of Biscay.  The foraminifera-based
reconstructions were placed into a
temporal framework using 137Cs, heavy
metal concentrations, and 210Pb-derived
sediment accumulation rates. The
resulting relative sea-level curves were
integrated with the Santander tide-gauge
data providing a regional relative sea-
level rise of 1.9 mm yr-1 for the 20th
century. This result contrasts with the
almost negligible sea-level rise proposed
for the same area during the 19th century
by Leorri et al. (2008b), supporting the
idea of a global acceleration in the rates
of sea level at the turn of the 19th century
linked with human-induced climatic
change.
It is, therefore, desirable to obtain
new high-resolution sea-level data from
the Holocene, as they represent the
fundamental basis for comparison with
the historical and present changes. They
provide a benchmark against which one
must measure the additional sea-level rise
that has occurred over the last 100-150
years (Church and White, 2006; Holgate,
2007). Available sea-level data from the
North Atlantic Ocean provide a broad
picture of fast sea-level rise since 15,000
cal yr BP (from 100-120 m below current
level) until 6000 or 5000 cal yr BP when
sea level reached its present position.
Since then, sea level has been relatively
stable (Lambeck, 1997).
We hypothesize that foraminiferal
and sedimentological analysis combined
with 14-C dating of the Holocene
estuarine infilling would provide high-
quality data from the Bay of Biscay to
examine the issue of Holocene changes in
the rate of relative sea-level rise. In this
paper, we use the modern inferred
relationship of foraminiferal assemblages
with elevation derived from three
different estuarine areas (Leorri et al.,
2008a) to provide a Holocene sea-level
curve. However, we argue that a single
estuarine area could reflect local rather
than regional forcing factors.
Consequently, we study here multiple
Holocene cores from two estuaries
(Bilbao and Urdaibai).
Study area
This study was conducted in two
estuaries with similar mesotidal ranges
(mean tidal range: 2.5 m; Leorri et al.,
2008b). The Bilbao estuary was
originally the most extensive estuarine
area on the Cantabrian coast of northern
Spain. The modern estuary is 15 km long
and is formed by the tidal part of the
Nervion river, although four other rivers
(Kadagua, Asua, Galindo and Gobelas)
discharge into the main course. Today the
Bilbao estuary is a largely artificial
system which bears little resemblance to
the original estuary. It has been calculated
that the total amount of the original
estuarine surface lost through human
128
GEOGACETA, 46, 2009
Paleoclimatología
E. Leorri y A. Cearreta
activity is approximately 1000 ha (Leorri
and Cearreta, 2004). On the other hand,
the Urdaibai estuary (Urdaibai Biosphere
Reserve) is formed by the tidal part of the
Oka river, covers an area of 765 ha, and
occupies the flat bottom of the 11.6 km
long, 1 km wide alluvial valley (Leorri et
al., 2008b).
Materials and methods
In order to establish the general
framework of the relative sea-level rise
during the Holocene at the regional level,
61 samples recovered from 20 boreholes
and one trench were selected and
analyzed for sedimentological and
micropalaeontological contents and
radiocarbon dated. Samples were chosen
as representative of different estuarine
subenvironments and elevations. All data
are presented relative to current mean
tidal level at the Bilbao tide gauge; 2.4 m
above local ordnance datum (lowest tide
at the Bilbao Harbour on 27th September
1878). The boreholes were drilled using a
rotary drill  that produced a core
approximately 10 cm in diameter. In each
case, the borehole terminated in
Cretaceous basement.
S e d i m e n t o l o g i c a l  a n d
Micropalaeontological sample preparation
followed standard methods and are
described in Leorri and Cearreta (2004).
Beta Analytic Inc. (USA) and NSF-AMS
Facility at the University of Arizona (USA)
carried out radiocarbon dating on forty
five shell samples, four samples of
foraminiferal tests (Ammonia tepida),
eleven wood samples, and one bone
sample. Twenty seven of them were large
enough for radiometric analysis and C-14
content was quantified by measuring
emanating radiation which occurs during
the decay process. Of those, four samples
contained less than 1 gr of final carbon
and were analyzed with extended
counting to enhance precision. The other
thirty four samples were very small and
required direct atomic counting using an
accelerator mass spectrometer. All
radiocarbon dates were calibrated into
calendar years before present (cal yr BP)
using CALIB 5.0.1 (Stuiver et al., 2005);
Marine04 curve was used for marine
samples and IntCal04 was used for non-
marine samples (i.e., wood and bone
samples). The dates obtained on shell
material have been also corrected for the
marine reservoir effect (apparent surface-
water age), which has been estimated to
be around 400 years on the Bay of Biscay
(Leorri and Cearreta, 2004).
Sea-level indicative meaning and sources
of error
The vertical position of a sea-level
index point (SLIP) relative to mean tidal
level (MTL) is calculated as (Massey et
al., 2008):
SLIP = H – D – I + C + A,
where H is the height of the core top
relative to MTL, D is the depth of the
sample in the core, I is the height of
deposition of the sample (indicative
meaning) relative to MTL as inferred
from table I, C is the core compaction,
and A is the autocompaction. Derived
rates of sea-level rise were calculated
using linear regression analysis.
Based on the sedimentological and
micropalaeontological contents of
modern environments and their
relationship with elevation described by
Leorri et al. (2008a; 2008b), Cearreta et
al.  (2008), and Leorri and Cearreta
(2009b), we infer the environment in
which the samples originated («indicative
meaning» of the SLIPs; Table I)
following Gehrels et al. (2006), Ruiz et
al. (2005), and Mauz and Bungenstock
(2007). Although samples close to the
contact with the basement, basal SLIPs,
would be less prone to error (e.g., less
autocompaction) as it happens with basal
peat samples, the materials overlaying the
Cretaceous basement are always fluvial
gravels that in many cases have been
reworked during the Holocene sea-level
rise (Leorri and Cearreta, 2004)
seemingly precluding the use of these
materials for sea-level reconstructions.
There are different sources of error
when creating SLIPs, some of them
being more relevant than others. The
vertical  error calculation follows
Shennan and Horton (2002) (total error=
√(e1
2+e2
2+...en
2)) and includes the sum of
field levelling (± 0.02 m), sample
levelling within the core (± 0.05 m),
modern vertical distribution of the
different environments (different
environments have different tidal ranges).
However, from all possible sources,
Fig. 1.- Plot of sea-level index point points, showing depth within the core (m) against eleva-
tion above the contact with the basement (m) and calibrated age for some samples used to
correct for autocompaction. Different symbols represent groups of similar radiocarbon ages
(only two groups are presented). Grey bars indicate the age range for samples represented as
dots and boxes (white and black) located in the middle area. White boxes are muddy samples
(more autocompacted) and black boxes are sandy samples (less autocompacted). Dots are
sandy samples of younger age. Greater elevation above the basal contact correlates with grea-
ter depth as a result of the autocompaction. Estimated vertical errors associated with different
depositional environments are also indicated.
Fig. 1.- Gráfico de los puntos indicadores del nivel marino, mostrando la profundidad dentro del
sondeo (m) frente a su elevación respecto al contacto basal (m) y la edad calibrada de algunas de
las muestras usadas para corregir la autocompactación. Los diferentes símbolos representan
grupos similares de edades de radicarbono (sólo se presentan dos grupos). Las barras grises
indican el rango de edades para las muestras representadas como puntos y cajas (blancas y
negras) situadas en la zona media de la gráfica. Las cajas blancas son muestras fangosas (más
autocompactadas) y las cajas negras son muestras arenosas (menos autocompactadas). Los
puntos son muestras arenosas de edad más joven. Las mayores elevaciones respecto a los
contactos basales correlacionan con las mayores profundidades como resultado de la
autocompactación. Los errores verticales asociados a los diferentes medios deposicionales
también aparecen indicados.
129
GEOGACETA, 46, 2009
Sociedad Geológica de España
Anthropocene versus Holocene sea-level rise rates in the southern Bay of Biscay
autocompaction (loss of porosity due to
the load of overlying sediments)
introduces the greatest vertical errors,
especially from unconsolidated peat
sequences (Gehrels, 1999). Boreholes
obtained in both the Bilbao and Urdaibai
estuaries indicate that the depth of the
stratigraphic contact between the
basement and the Quaternary sediments
greatly varies between 4 and more than
30 m from the upper to the lower estuary,
respectively. These sediments are
composed mainly by gravels and coarse
sands at the base; they vary however in
the mud (silt and clay) content, with the
presence of some organic-rich lenses
(Leorri and Cearreta, 2004). Thus,
although minerogenic sediments
dominate and the sand content is very
high, the great length of some cores and
the variable mud content suggest that
additional accommodation space due to
autocompaction can be responsible for at
least part of the observed sea-level rise.
Hence, in order to address
autocompaction we have directly
compared samples with statistically
similar radiocarbon ages recovered at
different elevations above the basement-
Quaternary contact (Fig. 1), following
Gehrels (1999). One limitation of our
approach is that samples do not
necessarily represent equivalent
depositional environments and, therefore,
this model should be considered
carefully. Nevertheless, the results
obtained allow to infer reasonably well
the actual extent of autocompaction.
Based on the vertical errors
mentioned and the substantial age errors
(true ages lie somewhere in a time span
between 100 and over 650 years) the
obtained sea-level curve has to be
interpreted in terms of metre scale
vertical resolution and sub-millennial
scale age resolution.
Results and Discussion
Figure 2 illustrates observations of
sea-level change for the past ca. 9000
years from the Bilbao and Urdaibai
estuaries based on the sand content and
foraminiferal assemblages. From the new
61 samples analyzed here as SLIPs, forty
five can be considered good SLIPs,
ranging from ca. 8500 cal yr BP to ca. 200
cal yr BP. Fourteen samples plot
significantly lower or higher than expected
and are considered to be the result of
reworked materials, and therefore
discarded as SLIPs (black squares in Fig.
2). Similar age inversions have been
reported by Cearreta and Murray (2000)
when attempting to date transgressive
surfaces in the Bilbao and Santoña
estuaries. Three fluvial samples are used
as limiting dates (i.e., sea level must be
below these points) although they are not
considered as valid sea-level index points.
SLIPs from both estuaries exhibit a very
good agreement and, consequently, will be
discussed together. The overall trend
exhibits two main phases: 1- fast relative
sea-level rise prior to 7000 cal yrs BP, that
ranges between 9 to 12 mm yr-1; and 2- a
relative sea-level rise of 0.7 mm yr-1 since
ca 6700 cal yr BP until 19th century (Fig.
2). This trend represents both sea-level rise
and vertical land movements,
unfortunately, no research on Holocene
neotectonic movements has so far been
undertaken in this region, although such
causes have been invoked to explain
aspects of the evolution of this margin
during the Neogene and Pleistocene
(Mary, 1983).
The initial phase of sea-level rise
presents few and disperse samples and,
therefore, they could provide a
misleading interpretation. This probably
responds to a highly energetic
depositional environment. The estimation
of relative sea-level rates derived from all
the SLIPs younger than 7000 cal yr BP
provide a figure of 0.7 mm yr-1 for the pre-
anthropogenic sea-level rise, in
agreement with estimations of the
eustatic contribution from melting of
land-ice base, although this is still a
matter of content (see Gehrels et al.,
2006). This figure could be even smaller
if autocompaction has not been fully
corrected. It is now generally recognised
that sea level has been relatively stable
over the last 7000-6000 years (Church et
al., 2008) as found here. However, the
rate of sea-level rise changes abruptly
when recent relative sea-level variations
are analyzed. Leorri and Cearreta (2009a)
obtained a RSL curve based on
foraminiferal transfer functions from salt
marshes coupled with local and regional
tide-gauge records that provided a regio-
nal sea-level rise of 1.9 ± 0.3 mm yr-1
since 1920, seemingly related to the
anthropogenic impact of the global
warming. Although isostatic corrections
should be performed (Lambeck, 1997), a
similar vertical movement will be applied
to both pre-anthropogenic and
anthropogenic rates and so the difference
between them should remain constant.
Although these results are
encouraging, in order to better constrain
pre-anthropogenic sea-level changes,
further analysis has to be done to narrow
down current reconstruction errors of the
Holocene reconstructions.
Acknowledgements
Dr. Leorri was supported by the
contract SFRH/BPD/44750/2008
Fig. 2.- Plot of Holocene sea-level index points from the Bilbao and Urdaibai estuaries, showing
calibrated age against depth relative to present mean tide level (m). Vertical and age errors are
also represented. Black boxes indicate samples not included in the trend estimations.
Fig. 2.- Gráfico de los puntos indicadores del nivel marino en los estuarios de Bilbao y Urdaibai,
mostrando las edades calibradas frente a la profundidad relativa al nivel medio de la marea
actual (m). Los errores de elevación y edad también se presentan. Los cuadrados negros indican
las muestras no incluidas en las estimaciones de las tendencias.
130
GEOGACETA, 46, 2009 E. Leorri y A. Cearreta
Geología Paleoclimatología
(Fundação para a Ciência e a Tecnologia,
FCT; Portugal). Nieves González
provided technical support. This research
was funded by the following projects:
GIC07/32-IT-332-07, Acção Integrada
Luso-Espanhola nº E-20/80/ Acción
Integrada Hispano-Portuguesa HP2007-
0098 (Assessing palaeoenvironmental
proxies in the Iberian coast: marsh
foraminifera as sea-level indicators) and
Estudio de las variaciones recientes del
nivel marino en la costa vasca en el marco
del proyecto K-Egokitzen-Cambio
climático: impacto y adaptación (Etortek).
This paper is also a contribution to IGCP
Project 495 («Late Quaternary Land-
Ocean Interactions: Driving Mechanisms
and Coastal Responses») and to the
Northwest Europe working group of the
INQUA Commission on Coastal and
Marine Processes.
References
Cearreta, A., Leorri, E., Iriondo, I.,
González, M.J.B. y Aristondo, O.
(2008). Geogaceta, 45, 67-70.
Cearreta, A. y Murray, J.W. (2000). The
Holocene, 10, 155-159.
Church, J.A. y White, N.J. (2006).
Geophysics Research Letters, 33,
L01602.
Church, J.A., White, N.J., Aarup, T.,
Wilson, W.S., Woodworth, P.L.,
Domingues, C.M., Hunter, J.R. y
Lambeck, K. (2008). Sustainability
Science, 3, 9-22.
Gehrels, W.R. (1999). Quaternary
Research, 52, 350-359.
Gehrels, W.R., Szkornik, K., Battholdy,
J., Kirby, J.R., Bradley, S.L., Marshall,
Table I.- Indicative meaning of lithofacies based on surveys of modern depositional environ-
ments, plant zones and associated foraminiferal assemblages. Fresh water tidal mud flat upper
limit and subtidal lower limit have been overextended considering that a) both types of sam-
ples represent upper and lower bounds respectively for the mean tide level (MTL), and b) in
the past they could have presented greater ranges than today.
Tabla I.- Valor indicativo de las litofacies en base a muestreos de los ambientes deposicionales
actuales, zonación vegetal y asociaciones de foraminíferos. Los límites de los ambientes de
marisma mareal de agua dulce y ambiente submareal han sido extendidos considerando que a)
ambos ambientes representan los límites superior e inferior del rango mareal y b) en el pasado
esos rangos han podido ser mayores.
W.A., Heinemeier, J. y Pedersen, J.B.T.
(2006). Quaternary Research, 66, 288-
302.
Holgate, S.J. (2007). Geophysics
Research Letters, 34, L01602.
Lambeck, K. (1997). Palaeogeography,
Palaeoclimatology, Palaeoecology,
129, 1-22.
Leorri, E. y Cearreta, A. 2004. Marine
Micropaleontology, 51, 75-94.
Leorri, E. y Cearreta, A. (2009a). Scientia
Marina, 73(2), 287-296.
Leorri,  E. y Cearreta, A. (2009b).
Continental Shelf Research, 29, 1226-
1239.
Leorri, E., Cearreta, A. y Horton, B.P.
(2008a). Geobios, 41, 787-797.
Leorri, E., Horton, B.P. y Cearreta, A.
(2008b). Marine Geology, 251, 60-74.
Mary, G. (1983). Trabajos de Geología,
13, 3-35.
Massey, A.C., Gehrels, R.W., Charman,
D.J., Milne, G.A., Pelier, W.R., Lambeck,
K. y Selby, K.A. (2008). Journal of
Quaternary Science, 23, 415-433.
Mauz, B. y Bungenstock, F. (2007).
Marine Geology, 237, 225-237.
Ruiz, F., González-Regalado, M.L.,
Pendón, J.G., Abad, M., Olías, M. y
Muñoz, J.M. (2005). Quaternary
International, 140-141, 21-36.
Shennan, I. y Horton, B.P. (2002).
Journal of Quaternary Science, 17,
511-526.
Stuiver, M., Reimer, P.J. y Reimer, R.
(2005). Radiocarbon calibration
program revision 5.0.1. Copyright
1986-2005.


Anthropocene versus Holocene relative sea-level rise rates in the southern Bay of Biscay

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=EE40E54AB671D6519D240CC5CEC6AA8A?doi=10.1.1.1068.550&rep=rep1&type=pdf

Anthropocene versus Holocene relative sea-level rise rates in the southern Bay of Biscay

Abstract

Similar works

Full text

Available Versions

CiteSeerX