Paleoclimatic evolution of the Galician continental shelf (NW of Spain) during the last 3000 years: from a storm regime to present conditions

doi:10.1016/j.jmarsys.2004.07.015

Journal of Marine Systems

Volume 54, Issues 1–4, February 2005, Pages 245-260

https://doi.org/10.1016/j.jmarsys.2004.07.015 Get rights and content

Abstract

A multiproxy study of the sedimentary record carried out on gravity core CGPL00-1 retrieved from the outer Galician continental shelf (NW of Spain) has allowed us to establish the main climate fluctuations affecting the region during the Upper Holocene. Grain size, TOC, C/N ratio, biogenic opal and planktonic foraminifera are the main analysed parameters. Lithology and grain size distribution lead to identify two sedimentary sequences: a lower half mainly composed by glauconitic sand and a muddy upper half. A chronology has been established based on three AMS radiocarbon ages, 907 cal. BC, 898 cal. BC and 1399 AD, and the aforementioned sedimentary sequences. The obtained radiocarbon ages are the first dated sediment samples for the Galician continental shelf. Geochemical markers show different trends in both sequences: low and/or fluctuating values in the sandy sequence and high and relatively constant values in the upper muddy sequence. The whole sandy interval is interpreted to be a nearly instantaneous deposit from a distal storm ebb current. The muddy interval was deposited in a stable and low-energy marine environment, similar to that found on the present outer shelf. The shift from a storm-dominated shelf to a low-energy environment took place synchronous with the Subboreal/Subatlantic transition, when an increase in storminess appears related to climatic instability. Transitional planktonic foraminiferal assemblage dominates the whole record, although a change to more abundant cold water species at 1420 AD, may relate to an intense upwelling pulse, probably reinforced by colder atmospheric temperatures during the Little Ice Age (LIA). Despite the presence of an upwelling regime since 1420 AD, lesser amount of opal has accumulated in the outer shelf due to enhanced offshore transport and stronger remineralization.

Introduction

Understanding factors affecting climate change in the Holocene is of considerable importance: the topic being the subject of much current research and debate in relation to future climate change. Several climatic oscillations have characterized the last 3000 years. The most important has been identified at the Subboreal/Subatlantic transition, at around 850 cal. BC (van Geel et al., 1996), characterized by a shift from a relatively warm climate to cool and wet conditions in mid-latitudes (Kilian et al., 1995, van Geel et al., 1999). The first warm period in the last 3000 years coincides with the Roman Warm Period peaking at 100 AD (Bianchi and McCave, 1999) with a return to cold conditions after the fifth century in the Dark Ages (Lamb, 1995). In the last millennium, the best identified climatic fluctuations are the Medieval Warm Period (MWP), 900–1250 AD (Lamb, 1995), and the Little Ice Age (LIA), 1350–1800 AD (Stuiver et al., 1995).

These climatic oscillations have been registered by several authors in a large variety of records, for example in Greenland ice cores (Meese et al., 1994, O'Brien et al., 1995, Kreutz et al., 1997, Dahl-Jensen et al., 1998), North Atlantic deep sea cores (Keigwin, 1996, Bond et al., 1997, Bianchi and McCave, 1999), subtropical deep sea cores (deMenocal et al., 2000), lake sediments (Campbell et al., 1998), peat bogs (Barber et al., 2000) and tree rings (Edwards et al., 2000). However, paleoclimate records from shallow marine areas are very scarce. Records from such areas provide decisive information connecting continental with deep sea records. High sedimentation rates in these areas allow for detailed reconstructions in terms of temporal resolution, but the establishment of an accurate chronology in these environments is difficult thus hindering correct interpretation of the sedimentary record.

The present study is based on material from a 96 cm gravity core (CGPL00-1) retrieved from the outer Galician continental shelf (NW of Spain, 42°5′15.115″N, 9°3′46.380″W, 130.8 m water depth, Fig. 1a). Grain size, organic carbon, organic carbon/total nitrogen ratio, biogenic opal and planktonic foraminifera are the main parameters constituting this multiproxy study. The aim of the present work is to integrate these geochemical and micropaleontological proxies in order to reconstruct environmental factors (water dynamics, storminess, water column productivity, etc.) and their evolution during the last 3000 years in the western Galician outer continental shelf.

Section snippets

Regional setting

The study area (Fig. 1a) extends between 41°53′ and 42°53′ latitude N and 8°35′ and 9°32′ longitude W, and represents a climatic province in mid-latitude region, strongly influenced by the position of the polar front. The oceanography of the western Galician coast is affected by wind-driven upwelling that is common along the eastern boundary of the North Atlantic between 10°N and 44°N (Wooster et al., 1976). The seasonal evolution of the Iberian upwelling is closely related to the large-scale

Materials and methods

The gravity core used in this work was recovered on board the B/O Mytilus during a cruise in May 2000. The core was sealed just after collection and kept in storage at 4 °C until analyses were performed in the laboratory. The core was split longitudinally in two sections and 1 cm thick slices were removed for radiographical analyses with a Cabinet X-ray System (Faxitron Series, Hewlett-Packard). After splitting, the core was visually described and sampled. One section was used for

Core description

Gravity core CGPL00-1 presents two well-differentiated sections (Fig. 2a). An intense bioturbation was observed throughout the core, especially in some critical intervals around 80 and 70 cm. The lower half consists mainly of sand overlaying a basal 2 cm interval of bioclastic gravel. Internal laminations were observed in this sandy interval, although sometimes obscured by bioturbation. The lamination is clearly visible on X-ray radiographies from 91 to 79 cm. Glauconite mean percentage in the

Discussion

Sharp lithological contrasts recorded in the core and the trends of most measured variables (TC, TOC, TN, opal and foraminiferal abundance) indicate the existence of two distinct intervals with different environmental conditions in the study region during the last 3000 years. Throughout the sandy interval low TC, TN and opal levels are recorded, whereas in the muddy interval high values of these parameters occur from about 2800 cal. BP to present.

The age model previously discussed indicates

Conclusions

During the last 3000 years, two periods with contrasted environmental conditions have been identified in the western Galician continental shelf. The most pronounced environmental change took place at 2850 cal. BP, coinciding with the Subboreal/Subatlantic transition. This transition represents a climate instability period that was accompanied by high storminess in mid-latitudes. Sedimentation on the western Galician shelf is interpreted to have been controlled by this storm regime. Strong

Acknowledgements

We thank S. Rúa-Santerbás and M. Martínez-García their help with sample processing and the Mytilus crew for help in core collection. A.W. Dale and K. Rea were of great help in making language corrections. We are indebted to M. Pérez-Arlucea for her helpful critical comments on this paper. Very constructive reviews by F.J. Sierro and M. Leeder are sincerely acknowledged. R.G.A. and P.B. acknowledge Ministerio de Educación, Cultura y Deportes and Xunta de Galicia for doctoral grants. This work

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Paleoclimatic evolution of the Galician continental shelf (NW of Spain) during the last 3000 years: from a storm regime to present conditions

Abstract

Introduction

Section snippets

Regional setting

Materials and methods

Core description

Discussion

Conclusions

Acknowledgements

Marine Geology

Quaternary Science Reviews

Marine Geology

Marine Geology

Marine Micropaleontology

Geochimica et Cosmochimica Acta

Marine Geology

Quaternary Science Reviews

Marine Geology

Chemical Geology

Deep-Sea Research

Continental Shelf Research

Oceanologica Acta

Continental Shelf Research

Global and Planetary Change

Deep-Sea Research

Quaternary Research

Quaternary Science Reviews

Proximality trends in modern storm sands from the Helgoland Bight (North Sea) and their implications for basin analysis

Senckenbergiana Maritima

An ecological, zoogeographic and taxonomic review of recent planktonic foraminifera

Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland

Nature

Coastal upwelling of the Rías Bajas, Galicia, northwest Spain: I. Hydrographic studies

Rapports et Proces Vervaux des Reunions-CIESM

Size-fractionated primary productivity and biomass in the Galician shelf (NW Spain): net plankton versus nanoplankton dominance

Scientia Marina

A pervasive millenial-scale cycle in North Atlantic Holocene and glacial climates

Science

On the organic carbon maximum on the continental slope of the eastern Arabian Sea

Journal of Marine Research

Late Holocene ∼1500 yr climatic periodicities and their implications

Geology

Paleoceanographic reconstructions from planktonic foraminifera off the Iberian Margin: temperature, salinity and Heinrich events

Paleoceanography

Past temperatures directly from Greenland ice sheet

Science

Coherent high-and low-latitude climate variability during the Holocene warm period

Science

The last 3000 years in the Ría de Vigo (NW Iberian Margin): climatic and hydrographic signals

Holocene

Processes controlling the organic carbon content of open oceans sediments

Paleoceanography