Petrological inferences on the evolution of magmas erupted in the Andagua Valley,Peru (Central Volcanic Zone)

Major and trace element and Sr and Nd isotope data is presented from the Andagua valley scoria cone and lava field (15°32′ S 72°19′ W), Southern Peru in the northernmost part of the Central Volcanic Zone (CVZ). The rocks are all quite evolved in composition (SiO₂ = 55–64 wt.%) and classify as benmoreites, latites and few mugearites and trachytes. Samples are characterized by high Na₂O (4.2–5.2 wt.%), Sr (600–1300 ppm), Ba (800–1600 ppm). The main difference between the benmoreites and latites is in the Na₂O content that reach the highest so far reported from CVZ for these SiO₂ concentrations. The rocks are generally nearly aphyric but latites and trachytes are more porphyritic. Amphibole microphenocrysts generally are only present in latites and trachytes. The difference between benmoreite and latite samples is reflected in lower P₂O₅ and Zr content of the latite samples documenting the existence of two compositional different parental magma types. The investigated volcanic activity spans the Pleistocene to Recent with the historic activity concentrated in the area just south of Andagua. Combined relative stratigraphy, petrography and geochemistry define volcanic units and demonstrate that rocks from Chilcayoc Grande, Chilcayoc Chico 2, Jenchana, Sucna 1 and Chilcayoc Chico 1 represent the most recent volcanic activity. The main trend samples, each form a co-magmatic group resulting in sub-parallel trends in many variations diagrams. It is furthermore shown that these trends point towards calculated mixing lines relating the individual units through a binary mixing process, thus indicating a two stage evolution. In the case of Jenchana, Sucna 1 and Chilcayoc Chico 1, the samples define positive correlation trends in the Sr vs. Rb diagram that can be extrapolated back towards origo indicating nearly perfect incompatibility of Sr and Rb. This together with generally high Sr/Y (50–105) and low Y content (< 16 ppm) suggest lack of plagioclase fractionation and residual garnet in the source and is taken as evidence for relatively high pressure (lower crustal) origin of the mixing event. The amphibole bearing samples form individual co-magmatic groups that cannot be related to each other. This means that the amphibole bearing samples originates from different magmas. The lavas of the Ninamama group are comparable in age to the main trend samples but different in petrography and composition, why the two compositional different magmas must have existed within a small confined area within a limited time span.     

Magma evolution of Quaternary minor volcanic centres in southern Peru,Central Andes

Minor centres in the Central Volcanic Zone (CVZ) of the Andes occur in different places and are essential indicators of magmatic processes leading to formation of composite volcano. The Andahua–Orcopampa and Huambo monogenetic fields are located in a unique tectonic setting, in and along the margins of a deep valley. This valley, oblique to the NW–SE-trend of the CVZ, is located between two composite volcanoes (Nevado Coropuna to the east and Nevado Sabancaya to the west). Structural analysis of these volcanic fields, based on SPOT satellite images, indicates four main groups of faults. These faults may have controlled magma ascent and the distribution of most centres in this deep valley shaped by en-echelon faulting. Morphometric criteria and ¹⁴C age dating attest to four main periods of activity: Late Pleistocene, Early to Middle Holocene, Late Holocene and Historic. The two most interesting features of the cones are the wide compositional range of their lavas (52.1 to 68.1 wt.% SiO₂) and the unusual occurrence of mafic lavas (olivine-rich basaltic andesites and basaltic andesites). Occurrence of such minor volcanic centres and mafic magmas in the CVZ may provide clues about the magma source in southern Peru. Such information is otherwise difficult to obtain because lavas produced by composite volcanoes are affected by shallow processes that strongly mask source signatures. Major, trace, and rare earth elements, as well as Sr-, Nd-, Pb- and O-isotope data obtained on high-K calc-alkaline lavas of the Andahua–Orcopampa and Huambo volcanic province characterise their source and their evolution. These lavas display a range comparable to those of the CVZ composite volcanoes for radiogenic and stable isotopes (⁸⁷Sr/⁸⁶Sr: 0.70591–0.70694, ¹⁴³Nd/¹⁴⁴Nd: 0.512317–0.512509, ²⁰⁶Pb/²⁰⁴Pb: 18.30–18.63, ²⁰⁷Pb/²⁰⁴Pb: 15.57–15.60, ²⁰⁸Pb/²⁰⁴Pb: 38.49–38.64, and δ ¹⁸O: 7.1–10.0‰ SMOW), attesting to involvement of a crustal component. Sediment is absent from the Peru–Chile trench, and hence cannot be the source of such enrichment. Partial melts of the lowermost part of the thick Andean continental crust with a granulitic garnet-bearing residue added to mantle-derived arc magmas in a high-pressure MASH [melting, assimilation, storage and homogenisation] zone may play a major role in magma genesis. This may also explain the chemical characteristics of the Andahua–Orcopampa and Huambo magmas. Fractional crystallisation processes are the main governors of magma evolution for the Andahua–Orcopampa and Huambo volcanic province. An open-system evolution is, however, required to explain some O-isotopes and some major and trace elements values. Modelling of AFC processes suggests the Charcani gneisses and the local Andahua–Orcopampa and Huambo basement may be plausible contaminants.     

Lead isotopic evidence for evolutionary changes in magma-crust interaction,Central Andes,southern Peru

Lead isotopic measurements were made on Andean igneous rocks of Jurassic to Recent age in Moquegua and Tacna Departments, southernmost Peru, to clarify the petrogenesis of the rocks and, in particular, to investigate the effect of crustal thickness on rock composition. This location in the Cordillera Occidental is ideal for such a study because the ca. 2 Ga Precambrian basement rocks (Arequipa massif) have a distinct Pb isotopic signature which is an excellent tracer of crustal interaction, and because geomorphological research has shown that the continental crust was here thickened drastically in the later Tertiary.Seven samples of quartz diorites and granodiorites from the Ilo and Toquepala intrusive complexes, and seven samples of Toquepala Group subaerial volcanics were analyzed for Pb isotopic compositions. The plutonic rocks range in age from Jurassic to Eocene; the volcanic rocks are all Late Cretaceous to Eocene. With one exception, the Pb isotopic ratios are in the ranges ²⁰⁶Pb/²⁰⁴Pb= 18.52–18.75, ²⁰⁷Pb/²⁰⁴Pb= 15.58–15.65, and ²⁰⁸Pb/²⁰⁴Pb= 38.53–38.74. The data reflect very little or no interaction with old continental material of the Arequipa massif type.Lead from four Miocene Huaylillas Formation ash-flow tuffs, two Pliocene Capillune Formation andesites and five Quaternary Barroso Group andesites has lower ²⁰⁶Pb/²⁰⁴Pb than that in the pre-Miocene rocks, but relatively high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb (²⁰⁶Pb/²⁰⁴Pb= 18.16–18.30, ²⁰⁷Pb/²⁰⁴Pb= 15.55–15.63, ²⁰⁸Pb/²⁰⁴Pb= 38.45–38.90). Tilton and Barreiro [9] have shown that contamination by Arequipa massif granulites can explain the isotopic composition of the Barosso Group lavas, and the new data demonstrate that this effect is evident, to varying degrees, in all the analysed Neogene volcanic rocks. The initial incorporation of such basement material into the magma coincided with the Early Miocene uplift of this segment of the Cordillera Occidental [32], and thus with the creation of a thick crustal root. The data strongly imply a relationship between crustal thickness and degree of crustal contamination of magmas in this area, but a rigorous relationship is not yet established.     

Causes of spatial compositional variations in Mariana arc lavas: Trace element evidence

New inductively coupled plasma mass spectrometry (ICP-MS) trace element data are presented on a suite of arc lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged from seamounts in the Central Island Province (CIP), the Northern Seamount Province (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples concluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source and an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along the Mariana arc can be better explained by the addition of spatially varying subduction components to a spatially varying mantle source. The data suggest that the subduction component in the CIP and VA is dominated by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is dominated by more silicic fluids derived from volcanogenic sediments as well as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge basalt source by loss of a small melt fraction, while the mantle wedge in the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of seamounts controls the arc and back-arc geometries, so the concomitant variation between subducted material and mantle composition may be no coincidence. The high field strength element (HFSE) data indicate a high degree of melting (∼ 25–30%) throughout the arc, ∼ 10% of which may be attributed to decompression and ∼ 20% to fluid addition.     

The composition of basaltic lavas from Bayuda,Sudan and their place in the cainozoic volcanic history of north-east Africa

Six new analyses of young basaltic rocks from the Bayuda field show the predominant rock types to be strongly undersaturated basanites and nepheline trachybasalts. Both types are believed to represent magmas of deep-seated origin. Similar rocks are widely distributed in north-east Africa but mildly alkaline to tholeiitic basalts were erupted along the eastern margin of the continent in early and late Cainozoic times, whereas along the Tripoli-Tibesti zone to the west mildly alkaline basalts were probably confined to the early Tertiary. The Tripoli-Tibesti zone was one of uplift and strongly tensional tectonics in the late Mesozoic and early Cainozoic, and at this time may have been a line of potential lithospheric rifting, but a period of quiescence followed and resurgence of activity in the late Cainozoic produced weaker tensional structures and more strongly alkaline basic magmas. The region between these two main zones of activity was characterized throughout by intermittent alkaline volcanicity and weak tectonism. Neverthless, fracture zones which apparently controlled the volcanicity are beginning to be recognized in this area. It is argued that African volcanic activity is related to linear, rather than circumscribed, areas of mantle activity. Possible connections with epeirogenic movements within the Alpine orogenic belt appear to have been neglected in the debate on the causes of African igneous activity.     

Episodic Cenozoic volcanism and tectonism in the Andes of Peru

Radiometric and geologic information indicate a complex history of Cenozoic volcanism and tectonism in the central Andes. K-Ar ages on silicic pyroclastic rocks demonstrate major volcanic activity in central and southern Peru, northern Chile, and adjacent areas during the Early and Middle Miocene, and provide additional evidence for volcanism during the Late Eocene. A provisional outline of tectonic and volcanic events in the Peruvian Andes during the Cenozoic includes: one or more pulses of igneous activity and intense deformation during the Paleocene and Eocene; a period of quiescence, lasting most of Oligocene time; reinception of tectonism and volcanism at the beginning of the Miocene; and a major pulse of deformation in the Middle Miocene accompanied and followed through the Pliocene by intense volcanism and plutonism. Reinception of igneous activity and tectonism at about the Oligocene-Miocene boundary, a feature recognized in other circum-Pacific regions, may reflect an increase in the rate of rotation of the Pacific plate relative to fixed or quasifixed mantle coordinates. Middle Miocene tectonism and latest Tertiary volcanism correlates with and probably is genetically related to the beginning of very rapid spreading at the East Pacific Rise.     

Drivers of peatland water table dynamics in the central Andes,Bolivia and Peru

Cushion plant dominated peatlands are key ecosystems in tropical alpine regions of the Andes in South America. The cushion plants have formed peat bodies over thousands of years that fill many valley bottoms, and the forage produced by the plants is critical for native and nonnative domesticated mammals. The sources and flow paths of water supporting these peatlands remain largely unknown. Some studies have suggested that glacier meltwater streams support some peatlands, and that the ongoing loss of glaciers and their meltwaters could lead to the loss or diminishment of peatlands. We analysed the hydrologic regime of 10 peatlands in four mountain regions of Bolivia and Peru using groundwater monitoring. Groundwater levels in peatlands were relatively stable and within 20 cm of the ground surface during the rainy season, and many sites had water tables 40–90 cm below the ground surface in the dry season. Topographic and groundwater elevations in the peatlands demonstrated that the water source of all 10 peatlands was hillslope groundwater flowing from lateral moraines, talus, colluvium, or bedrock aquifers into the peatlands. There was little to no input from streams, whether derived from glacier melt or other sources, and glacier melt could not have recharged the hillslope aquifers supporting peatlands. We measured the stable water isotopes in water samples taken during different seasons, distributed throughout the catchments, and the values are consistent with this interpretation. Our findings indicate that peatlands in the study region are recharged by hillslope groundwater discharge rather than stream water and may not be as vulnerable to glacial decline as other studies have indicated. However, both glaciers and peatlands are susceptible to changing thermal and precipitation regimes that could affect the persistence of peatlands.     

The Calipuy Formation of Northern Peru, and its relation to volcanism in the Northern Andes

The Calipuy Formation is a primarily volcanic sequence deposited during the period 33 to 10 m.y. ago when basaltic and andesitic volcanoes developed concomitant with a N60°E horizontal foreshortening of the Peruvian Andes. The axis of compression is inferred from both fault tectonics in and near the Calipuy sequence and from N30°W-striking fold axes within it. Dacitic domes younger than 10 m.y. unconformably overlie the Calipuy sequence.Basaltic and andesitic effusive rocks concurrently filled a basin which developed during volcanism. During subsidence 80% of the fill was provided by primary extrusive igneous material, whereas sedimentary rocks associated with the igneous assemblage account for only 20%.Limited chemical data indicate that Calipuy andesitic rocks are slightly richer in alkalies than the average Cenozoic andesite, but petrographic data show that they are similar to other andesites of this age found in similar environments in the Andes. However, the analyses are too few to make any real generalization concerning petrogenesis.     

Trace element transport rates in subduction zones: evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas

Trace element and Th, Sr and Pb isotope data for young lavas from the Tonga-Kermadec arc in the southwest Pacific suggest that geochemical variations in the lavas along the arc are linked to differences in the material being subducted beneath the arc. Lavas from the southern (Kermadec) segment of the arc have relatively radiogenic Pb isotope compositions, which reflects a contribution from subducted sediment. In contrast, much of the Pb in Tonga lavas is derived from the altered oceanic crust in the subducting Pacific Plate, and lavas from the northernmost Tonga islands of Tafahi and Niuatoputapu contain Pb and Sr derived from the subducted part of the Louisville Seamount Chain. The origin of the Pb in the lavas from these two islands can thus be traced to a point on the subducting slab, and this observation is used to estimate the rate at which trace elements are transported beneath the arc. Our calculations suggest that fluid-soluble elements such as U, Sr and Pb are transported from the subducted slab, across the mantle wedge and back to the surface in lavas over a period of approximately 2–3 Ma, and that magmas are erupted at the surface less than 350 ka after the melts are generated in the mantle wedge.     

Evidence of a microplate in the Southern Andes?

Paleomagnetic poles for Late Paleozoic-Early Mesozoic rocks exposed in stable and possible mobile zones of Western Argentina are reported. They suggest two interpretations.One of the interpretations suggests that a mobile zone situated at about 32.1°South 69.3°West, in the Central Andes, rotated about 60° clockwise after Late Paleozoic time.The other interpretation suggests that at least the Andean zone between 32.1° and 31.7° South is allochthonous and was situated in the Pacific, at tropical latitudes, in the Late Paleozoic. On this interpretation the accretion of this microplate to South America ocurred in Permo-Triassic times.     

Seismic Detection and Characterization of the Altiplano-Puna Magma Body, Central Andes

— The Altiplano-Puna Volcanic Complex (APVC) in the central Andes is the product of an ignimbrite “flare-up” of world class proportions (de Silva, 1989). The region has been the site of large-scale silicic magmatism since 10 Ma, producing 10 major eruptive calderas and edifices, some of which are multiple-eruption resurgent complexes as large as the Yellowstone or Long Valley caldera. Seven PASSCAL broadband seismic stations were operated in the Bolivian portion of the APVC from October 1996 to September 1997 and recorded teleseismic earthquakes and local intermediate-depth events in the subducting Nazca plate. Both teleseismic and local receiver functions were used to delineate the lateral extent of a regionally pervasive ～20-km-deep, very low-velocity layer (VLVL) associated with the APVC. Data from several stations that sample different parts of the northern APVC show large amplitude Ps phases from a low-velocity layer with Vs ≤ 1.0 km/s and a thickness of ～1 km. We believe the crustal VLVL is a regional sill-like magma body, named the Altiplano–Puna magma body (APMB), and is associated with the source region of the Altiplano–Puna Volcanic Complex ignimbrites (Chmielowski et al., 1999).¶Large-amplitude P–SH conversions in both the teleseismic and local data appear to originate from the top of the APMB. Using the programs of Levin and Park (1998), we computed synthetic receiver functions for several models of simple layered anisotropic media. Upper-crustal, tilted-axis anisotropy involving both Vp and Vs can generate a “split Ps” phase that, in addition to the Ps phase from the bottom of a thin isotropic VLVL, produces an interference waveform that varies with backazimuth. We have forward modeled such an interference pattern at one station with an anisotropy of 15%–20% that dips 45° within a 20-km-thick upper crust. We develop a hypothesis that the crust above the “magma body” is characterized by a strong, tilted-axis, hexagonally symmetric anisotropy. We speculate that the anisotropy is due to aligned, fluid-filled cracks induced by a “normal-faulting” extensional strain field associated with the high elevations of the Andean Puna.     

Drop size distribution and kinetic energy load of rainfall events in the highlands of the Central Rift Valley,Ethiopia

Abstract

Knowledge of rainfall characteristics is important for estimating soil erosion in arid areas. We determined basic rainfall characteristics (raindrop size distribution, intensity and kinetic energy), evaluated the erosivity of rainfall events, and established a relationship between rainfall intensity I and volume-specific kinetic energy KE_vol for the Central Rift Valley area of the Ethiopian highlands. We collected raindrops on dyed filter paper and calculated KE_vol and erosivity values for each rainfall event. For most rainfall intensities the median volume drop diameter (D₅₀) was higher than expected, or reported in most studies. Rainfall intensity in the region was not high, with 8% of rain events exceeding 30 mm h^-1. We calculated soil erosion from storm energy and maximum 30-min intensity for soils of different erodibility under conditions of fallow (unprotected soil), steep slope (about 9%) and no cover and management practice on the surface, and determined that 3 MJ mm ha^-1 h^-1 is the threshold erosivity, while erosivity of >7 MJ mm ha^-1 h^-1 could cause substantial erosion in all soil types in the area.

Editor Z.W. Kundzewicz; Associate Editor Q. Zhang     

Relationships between mineralization and silicic volcanism in the Central Andes

Studies of late Tertiary silicic volcanic centres in the Western and Eastern Cordilleras of the Central Andes show that three volcanic environments are appropriate sites for mineralization: (1) ring-fracture extrusions post-dating large calderas; (2) similar extrusions within ignimbrite shields; and (3) isolated, small silicic volcanoes. Subvolcanic tin mineralization in the Eastern Cordillera is located in silicic stocks and associated breccias of Miocene age. The Cerro Rico stock, Potosi, Bolivia, contains tin and silver mineralization and has an intrusion age apparently millions of years younger than that of the associated Kari Kari caldera. Similar age relationships between mineralization and caldera formation have been described from the San Juan province, Colorado. The vein deposits of Chocaya, southern Bolivia, were emplaced in the lower part of an ignimbrite shield, a type of volcanic edifice as yet unrecognized in comparable areas of silicic volcanism. The El Salvador porphyry copper deposit, Chile, is related to silicic stocks which may have been intruded along a caldera ring fracture. Cerro Bonete, Chile, provides a modern example of the volcanic superstructure which may have overlain isolated mineralized stocks and breccia pipes such as that of Salvadora at Llallagua, Bolivia.Existing models for the genesis of porphyry copper deposits suggest that they formed in granodioritic stocks located in the infrastructure of andesitic stratovolcanoes. Sites of porphyry-type subvolcanic tin mineralization in the Eastern Cordillera of Bolivia are distinguished by the absence of such andesitic structures. The surface expression of a typical subvolcanic porphyry tin deposit was probably an extrusive dome of quartz latite porphyry, sometimes related to a larger caldera structure. Evidence from the El Salvador porphyry copper deposit in the Eocene magmatic belt in Chile suggests that it too may be more closely related to a silicic volcanic structure than to an andesitic stratovolcano.The dome of La Soufriere, Guadeloupe is proposed as a modern analog for the surface expression of subvolcanic mineralization processes, the phreatic eruptions there suggesting the formation of hydrothermal breccia bodies in depth. Occurrence of mineralized porphyries, millions of years after caldera formation, does not necessarily indicate that intrusions and mineralization are not genetically related to the sub-caldera pluton, but may be a consequence of the long thermal histories (1–10 million years) of the lowermost parts of large plutons. Caldera formation can only inhibit mineralization by dispersal of ore metals when these are of magmatic origin, and ignimbrites should not be taken as being unlikely to be associated with porphyry mineralization. Whether ore metals are of wall rock or magmatic origin, the key to understanding the relationships between silicic volcanism and mineralization lies in the fractionation of trace elements within large zoned magma chambers during their igneous history, and their subsequent hydrothermal migration. Small, highly mineralized intrusions formed late in a caldera cycle (such as the Cerro Rico) may be due to the introduction of fresh supplies of mafic magma into the lower parts of the main pluton.     

Plio-Quaternary tectono-magmatic zonation and plate tectonics in the Central Andes

Seismic provinces in Peru and northern Chile may be defined in direct relation to the geometry of parts of Nazca plate that are being subducted under the Americas plate. Recent tectonism and calc-alkaline volcanism appear also to have a clear relationship to that same geometry of the subducted slab. Under northern and central Peru, the slab plunges at 10–15° to the northeast, and becomes almost horizontal farther east; at surface in the same region, recent calc-alkaline volcanism is absent and recent tectonics are mostly compressional. Under southern Peru and northern Chile, the slab plunges regularly at about 30° to the east; at the surface, calc-alkaline volcanism is still active and recent tectonism appears to be mostly extensional.     

Magnetic properties of the Lower Old Red Sandstone lavas in the Midland Valley,Scotland; palaeomagnetic and tectonic considerations

Thermal demagnetization studies of lavas in the Strathmore area of the Midland Valley, Scotland, support overall palaeomagnetic data found in previous studies of these rocks. Reduced directional scatter as compared to some earlier studies, is attributed to more effective demagnetization, resolving some of the directional complexity of previous studies. Combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism. The existence of almost antiparallel directional groups and field tests give supporting evidence for a “primary” (deuteric) origin of the main magnetization of these rocks. Additionally, a second remanence component having shallow reverse directions of magnetization, is attributed to later remagnetization in Old Red Sandstone time. The Midland Valley results are seen in conjunction with other Palaeozoic palaeomagnetic results and possible geodynamic implications are discussed.     

Upper Cenozoic volcanism in the Central Andes — Ages and volumes

Fifty-two new K-Ar dates for Upper Cenozoic volcanic rocks from north Chile and southwest Bolivia are presented, together with a compilation of previously available dates from this region. These dates are combined with calculations of volumes of lava and ignimbrite for a segment of the volcanic province (19°30′S to 22°30′S) to identify fluctuations in the level of volcanic activity during the last 24 Ma. Histograms of volumes against time have been plotted for each half-degree quadrant. In the southern half of the study area, there were peaks of activity in the periods 12 to 9 Ma and 6 to 3 Ma. In the northern half, a large proportion of the material was erupted in the period 6 to 0 Ma. This regional variation suggests that localized factors may govern the rate of volcanic output and complicates attempts at correlation with “pulses” of volcanic activity recognized elsewhere in the Pacific region. There is no conclusive evidence for volcanic episodes synchronous over such wide areas. A simple correlation between changes in spreading behaviour and changes in levels of volcanic activity is unlikely, in view of the complexity of the interactions at destructive plate margins. The rate of continental crust accretion from volcanic processes must be much less than that due to intrusive processes to account for the thickening of the Andean crust since the Jurassic.     

Trace element distribution during the reproductive cycle of female and male spiny and Pacific scallops, with implications for biomonitoring

Trace element concentrations and contents in gills, gonad, kidneys, mantle, muscle and remainder during the reproductive cycle of female and male spiny and Pacific scallops, from the Strait of Georgia, BC, Canada, were quantified by using ICPMS. The elements investigated were chromium, manganese, iron, cobalt, nickel, selenium, molybdenum, cadmium, tin and mercury. For all ten elements, the tissue distribution was to some extent influenced by species, sex and reproductive status. The implications of the present study in relation to the design of biomonitoring programmes are: (1) care should be taken to ensure an equal/constant sex composition when making interannual comparisons of pooled samples. Preferably the sexes should be monitored separately. (2) the practice of obtaining pooled samples in the interspawn phase is applicable only to monitoring long-term trends in contaminant levels, while the reproductive status should be heeded when studying short-term changes. (3) the present study confirms that direct temporal or spatial comparisons of absolute accumulated element concentrations are only valid intraspecifically.