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Anthropostratigraphy: new lithological units of the Quaternary controlled by human activity/ Antropoestratigrafia: nuevas unidades litologicas del Cuaternario controladas por la actividad humana.

1. Introduction

A recent geological mapping of the area occupied by the city of Barcelona (Fig. 1) raises a number of questions about the Holocene stratigraphy (Head et al., 2008) affected by human activity (Cires et al., 2009; Riba and Colombo, 2009).

There are few data on the detailed geology of urban habitats that have undergone repeated modifications during human settlement. The survey undertaken in the urban area of Barcelona enabled us to determine the main methods of study, outline the major geological problems, and define the specific stratigraphic units.

Sedimentary rock bodies accumulated during human settlement may be modified by anthropogenic activities (Kirwan et al., 2011, Kirwan and Murray, 2012). A number of areas such as the Mediterranean region have been inhabited without interruption for thousands of years. This is the case of Barcelona (Fig. 2), where remnants dating from pre-Roman times until the present have been found. These artefacts are usually mixed with natural sedimentary materials resulting from the geological sedimentary processes in urbanised areas.

The interaction between human activity and geology (Crutzen, 2002) in urbanised areas gives rise to the formation of diverse sedimentary lithosomes. These rock bodies must be characterized and arranged in stratigraphic units to enable us to reconstruct the geological and human history of these areas. It should be borne in mind that natural forces and human influences (Hooke, 2000; Syvitski et al., 2005; Wilkinson, 2005) can be either depositional or erosional.

This work constitutes a new stratigraphic approach to the study of human-induced sedimentary accumulations. Thus, in the upper Holocene, new units may be distinguished in urbanised areas: the anthropogenic stratigraphic units.

2. Quaternary chronostratigraphic units

In a very brief chapter, the 9 G, The International Stratigraphic Guide (ISG) states that "The basic principles used in subdividing the Quaternary into chronostratigraphic units are the same as for other Phanerozoic chronostratigraphic units, although the methods of time correlation may have a different emphasis. As in the case of other chronostratigraphic units, those of the Quaternary require boundary definitions and designation of boundary stratotypes" (Murphy and Salvador, 1999).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Given the large number of geological studies of the most recent period of the Earth affected by human presence, a multidisciplinary stratigraphic approach should be adopted to improve our understanding of Quaternary stratigraphy.

The Quaternary has been characterized by a series of ice ages interrupted by interglacial periods in middle-high latitudes of the Northern Hemisphere. It therefore seems reasonable that the geographical regions most affected by glacial deposits are those areas where the studies of Quaternary stratigraphy are more detailed and more frequent (Mangerud et al., 1974; Gibbard et al., 2005).

The issue of the Lithuanian Stratigraphic Guide (LSG) provides a general approach (Grigelis, 2002) that makes up for the scant reference to the Quaternary made by the International Stratigraphic Guide (ISG). New stratigraphic terms have been introduced into the LSG, where the Quaternary stratigraphic subdivision is established by applying a climatostratigraphic criterion. As a result, the climatostratigraphic units correspond to chronostratigraphic ones in the case of the Quaternary. Thus, the climatostratigraphic units are equivalent to the geochrono logical ones (Head et al., 2008).

In settlements older than 2000 years the lithostratigraphy provides evidence (artefacts) in differentiated units that are equivalent to chronostratigraphic ones. In other urbanised areas of Catalonia (NE, Spain) such as the city of Reus, similar artefacts have been found (Mir and Salas, 1979).

3. Holocene and Late Pleistocene stratigraphy

Despite its interest (Grigelis, 2002), the study of the Late Pleistocene (Head et al., 2008) and the Holocene (Gradstein 2006) poses a number of questions (Voisin, 2010; Walsh, 2006). Within a very short time span, the identification of small geological events separated by a few hundred or a few thousand years (Italian Commission on Stratigraphy, 2002) enables us to obtain accurate results. Correlation is not easy despite the existence of the many methods of study.

In some cases lacustrine (Copons and Bordonau, 1996), loess (Muhs et al., 1999), marine (Cacho et al., 2001) and ice-cores (Bjorck et al., 1998) present continuous successions that enable us to follow the classic stratigraphic methods. Sedimentary deposits usually occur in isolated outcrops and can be dated in accordance with the standard stratigraphic Quaternary scale, using radiometric, palaeontological and archaeological methods.

[FIGURE 3 OMITTED]

In recent years, attention has been increasingly focused on defining and correlating the human-influenced sedimentary accumulations. Archaeological studies (Butzer, 1980) have resulted in the use of prehistoric time scales despite the fact that little attention has been paid to the dating of the Late Quaternary history related to the role of human presence in urbanised areas. Our aim is to promote the use of the new lithoanthropogenic units in geoarchaeological studies and procedures (Rapp and Hill, 1998; Ghilardi and Desruelles, 2009; Zalasiewicz, 2008, 2010; Zalasiewicz, et al., 2008; Williams et al., 2011).

4. Anthropogenic stratigraphic units

As a result of the continuous settlement of humans in a given area, geological products have been affected by anthropogenic action in many situations. We seek to clarify the interactions among human activities and the coeval sedimentary environments. Many sedimentary bodies and all mechanical modifications of the rocks result from human activity. A new category of stratigraphic units is proposed: anthropogenic stratigraphic units.

These units are usually composed of sedimentary rock bodies with clear morphological boundaries. Thus, they are of limited extension because they infill voids and depressions. These depressions were produced by earlier settlements with the result that their infills, characterized by different lithologies, correspond to lithoanthropogenic stratigraphic units.

Human-influenced sedimentary bodies and rock mechanical modifications may be divided into three categories:

Natural lithoanthropogenic units (NAU). These units contain materials and artefacts of human origin although their formation is usually not directly caused by anthropogenic activity e.g. prehistoric axes, ceramic fragments ... etc. found in sedimentary horizons or in alluvial terraces. In this case, human remains and artefacts act as clasts (Fig. 3).

[FIGURE 4 OMITTED]

Induced lithoanthropogenic units (IAU). Sedimentation or erosion is produced by a combination of natural processes and human activity, e.g. the construction of a dam has a considerable influence on the geological behaviour of the river (Fig. 4).

Constructed lithoanthropogenic units (CAU). These correspond to sedimentary bodies that result from the direct influence of human activity (Fig. 5), e.g. accumulation of mine rejects and urban waste, infilled quarries, etc.

These three units are often present in urbanised areas where the IAUs are of particular interest since they play a major role in the history and development of the town. The Barceloneta district, for example, which is adjacent to the harbour, came into existence some centuries ago as a result of the interaction of longshore sandy currents and an artificial pier. Two sequences of beach progradation (Carsi, 1949) are interpreted as IAUs.

Other examples that offer new insights into some aspects of the history of Barcelona include the overflow of the Ramblas (Riera d'en Malla - intermittent stream) towards the low-lying areas of the Ciutat Vella district (Duran, 1963), which results in different levels of NAUs, and the infill of the moat of the Roman walls (Blasco et al., 1992), which gives rise to a CAU.

5. Geological mapping of urbanised areas

The geological maps of urbanised areas are the result of a study of the outcrops that are randomly distributed in the city area. However, a tentative correlation between well-known sites can be made despite the considerable difficulty of mapping extensive built-up areas,

Collaboration between geologists, historians and archaeologists is of paramount importance (Sole Sabaris, 1963). Thus, sedimentation analysis is the responsibility of geologists, whereas historians and archaeologists are concerned with establishing the age and origin of human remains and artefacts. Accordingly, human-made clasts found in Late Quaternary (Head et al., 2008) stratigraphic successions allow us to create anthropozones similar to geological biozones in older stratigraphic successions. Thus, these can only be applied to the accumulations in which there is a demonstrable human influence. A detailed knowledge of the history of a specific urban area enables us to deduce the succession of facts. Moreover the geological study of outcrops containing rock bodies or features related to human settlement allows us to determine the sequence of historical facts.

6. Discussion and concluding remarks

Human-influenced sedimentary bodies and rock mechanical modifications may be divided into three new categories of lithoanthropogenic units:

Natural lithoanthropogenic units (NAU) are defined in accordance with the following characteristics:

--Deposits of sedimentary origin controlled by flow variations that contributed to the accumulation of sediments.

--The boundaries of these units, which were generated by hydraulic variations, are attributed to autocyclic controls.

--These units contain materials and artefacts of human origin e.g. prehistoric axes, ceramic fragments ... etc. that can act as clasts (Fig. 3).

[FIGURE 5 OMITTED]

Induced lithoanthropogenic units (IAU) are defined in accordance with the following characteristics:

--This accumulation is usually controlled by artificially produced topographic changes, e.g. the medieval walls of the old city of Barcelona (Ciutat Vella). These walls were constructed in the middle of the thalwegs of former ravines and torrents, which resulted in large accumulations of sandy materials.

--The boundaries of these units, which are locally artificial and clearly defined, were a consequence of an obstacle placed across natural currents, e.g. the construction of the eastern pier of Barcelona harbour controlled the development of large sandy accumulation in recent historical times. This area is occupied by the district known as Barceloneta. In other cases, the boundaries are of sedimentary origin.

Constructed lithoanthropogenic units (CAU) are defined in accordance with the following characteristics:

--These units are always due to direct human activity.

--Their borders are clearly limited by excavations or deposition of urban waste, building debris, mine rejects, infilled quarries, dumping of rejected materials, etc.

--Locally, the units correspond to the remnants of buildings of other man-made constructions.

Despite the difficulty in defining these units, we suggest that researchers on the Quaternary consider the new lithoanthropogenic units proposed that can facilitate the study of recent sedimentary accumulations.

The NAUs display some artefacts that are older than the age of the deposit because they are reworked from former deposits

The NAUs and IAUs are reasonably well stratified and resemble classic lithostratigraphic units. However, the IAUs are usually associated with some kind of construction (dams, piers, etc.) or modification of the landscape (torrent diversion) by humans. Thus, their basal contact could be a non-conformity developed over the former natural deposits.

The CAUs commonly include materials that are the same or nearly the same age as the deposit. Locally, they may contain older materials dumped by humans. The CAUs usually lack stratification and are lithologically heterogeneous. These units consist mainly of man-made materials (bricks, rubble, garbage, etc.) and display a characteristic lack of stratigraphic continuity as a consequence of irregular periods of dumping.

In view of the above findings, the following considerations should be taken into account:

(A) It is essential to establish the stratigraphic succession of the materials found in wells and excavations. This is not always easy given that the elementary law of stratigraphy (the order of superposition indicates the chronological order of sedimentation) is often inapplicable. Reworked and introduced materials can usually be attributed to human settlement. Extreme caution should be adopted in the analysis of the outcrops in order to avoid misinterpretation.

(B) Correlation should be used to determine the chronostratigraphic unit to which the material found in a specific outcrop corresponds.

(C) Different episodes of continuous and/or interrupted settlement of the city should be determined. Human presence with its variable impact on pre-existing rock bodies should be more clearly defined.

http://dx.doi.org/10.5209/rev_JIGE.2013.v39.n2.42500

Acknowledgements

We are indebted to Drs. M. Lopez-Blanco, M. Marzo, P. Busquets, A. Arche and T. Lawton for their valuable comments and suggestions on the manuscript. We acknowledge the Departament d'Universitats, Recerca i Societat de la Informacio, Generalitat de Catalunya, Grup de Qualitat, 2009-SRG-1198, and Proyecto CGL200913706-C03-03E/BTE of the Spanish Ministerio de Ciencia e Innovacion (MINCINN) for financial support.

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F. Colombo (1) *, O. Riba (1, +), S. Reguant (1), C. Virgili (2), L. Rivero (3)

(1) Departament d'Estratigrafia, Paleontologia i GeocienciesMarines. Facultat de Geologia. Universitat de Barcelona. c/Marti i Franques s/n. E-08028 Barcelona, Spain. colombo@ub.edu

(+) In memoriam

(2) Departamento de Estratigrafia. Facultad de Ciencias Geologicas. Universidad Complutense de Madrid. E-28040 Madrid, Spain.

(3) Departament de Geoquimica, Petrologia i Prospeccio Geologica. Facultat de Geologia. Universitat de Barcelona. c/Marti i Franques s/n. E-08028 Barcelona, Spain

* corresponding author

Received: 01/02/2013 / Accepted: 14/04/2013
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Title Annotation:texto en ingles
Author:Colombo, F.; Riba, O.; Reguant, S.; Virgili, C.; Rivero, L.
Publication:Journal of Iberian Geology
Date:Jul 1, 2013
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