Varve Series Research Articles

Diversity of deformation structures in varves of the Weichselian Pyrzyce Basin (NW Poland)

Within the sediments of the Pyrzyce Ice-dammed Lake (Polish Lowlands), a number of soft-sediment deformation structures (SSDS) in varve series were distinguished. The unique morphology of the ice-dammed lake—the occurrence of level IV overload and the presence of dead ice in the central zone (Lake Miedwie) and the marginal reservoir—had a decisive impact on the formation and nature of sedimentary structures. Dead ice melting and loss of support could be the cause of plastic deformation. The highest intensity deformation in layered sediments of both plastic and rigid types was observed in the marginal (highest) part of the reservoir. Their formation is associated with the melting of rooted lumps of dead ice in sub-aquatic conditions. The exception is deformation resulting from desiccation under subaerial conditions. The genesis of the deformation observed in rhythmically-layered varves is associated with the last stage of functioning and disappearance of the reservoir during the recession of an ice sheet of the Pomeranian phase of the Weichselian glaciation. These studies detail the current state of knowledge on the processes and factors responsible for the formation of soft structures occurring in glaciolimnic basin.

Insights into the Connaught sequence of the Timiskaming varve series from Frederick House Lake, northeastern Ontario

A sub-bottom acoustic profile survey encountered a mass transport deposit (MTD) bed, 5–7 m thick, interbedded within glaciolacustrine deposits of glacial Lake Ojibway at Frederick House Lake, Ontario, Canada. Analysis of the thickness patterns of rhythmic couplets in recovered core samples revealed that the Connaught sequence, the youngest of the Timiskaming varve series, immediately underlie and overlie the MTD. Comparison to regional published varve series reveals two possible interpretations for the varve numbering: varve (v) 2066 to v2115, which requires the inference of a 55 varve year (vyr) disconformity just below the Connaught sequence, while the alternative numbering, v2011a to v2060a (a = alternative), extends continuously from older varves. Circumstantial evidence supporting the alternative numbering is as follows: (i) the uncertainty of applying a common 55 vyr disconformity to three varve series located up to 23 km apart and which otherwise exhibit closely matching thickness plots; (ii) the lack of evidence of an erosive unconformity in the sub-bottom acoustic profiles from Frederick House Lake; and (iii) the uncertain varve count within a key part of the Matagami series, located about 300 km away and from which the 55 vyr disconformity is extrapolated. At Frederick House Lake, the alternative numbering indicates that the maximum position of the Cochrane ice advance and the Connaught varves may be, in effect, contemporary in age. More broadly, the alternative numbering indicates that the youngest known varve that formed before the terminal drainage of glacial Lake Ojibway is v2074a rather than v2129 in the original numbering.

A 725‐year integrated offshore terrestrial varve chronology for southeastern Sweden suggests rapid ice retreat ~15 ka BP

The Swedish Varve Chronology is an unparalleled tool for linking the deglacial history of Sweden with associated palaeo‐environmental change at an annual time scale, and it forms part of Sweden's cultural heritage. A full deglacial chronology connected to the present day does not yet exist; a notable gap is in southeasternmost Sweden, where few varved records are successfully connected to reconstruct ice‐margin retreat. Deglaciation in southern Sweden covers both the climate transition to the Bølling warm period (~14.7 ka BP) and the ice‐margin transition from a subaqueous to terrestrial terminus. To facilitate investigations into the links between ice‐margin dynamics and abrupt climate change, we revisited the varve chronologies of southern Sweden. We digitized unpublished records, reanalysed existing varve thickness records, and obtained and analysed new varve series both on land and offshore. This combined approach has enabled us to refine and extend the existing south coast chronology east and 78 km northwards. Our new Skåne‐Småland chronology records 725 years of deglaciation, in addition to a younger floating chronology in parts. This chronology suggests that the glacial‐lake terminating Fennoscandian Ice Sheet in southern Sweden initially retreated northwards at ~110–160 m a−1 slowing to 60–70 m a−1 near the palaeo‐shoreline. Between today's mainland and the (now) island of Öland the retreat rates increase three‐ to fivefold. Ice‐margin retreat was initially oriented towards the north (as along the south coast), but later pivoted towards the northwest, signifying a landward retreat of terrestrial ‘Swedish’ ice that became divorced from the Baltic Sea ice‐sheet catchment. Our new 725‐year‐long varve thickness series reveals repeated multidecadal scale episodes of increased sedimentation. These likely signify phases of enhanced ice‐sheet melting that repeat and persist throughout the deglaciation of Skåne‐Småland.

Evidence of a strong paleoearthquake in ∼9.1 ka cal BP interpreted from mass transport deposits, western Quebec – northeastern Ontario, Canada

This paper reviews and expands upon stratigraphic and chronologic evidence supporting the interpretation of a strong, early postglacial paleoearthquake in western Quebec-northeastern Ontario, Canada. Event horizon mapping at Dasserat, Dufresnoy and Duparquet lakes, Quebec, coring at Chassignolle and Malartic lakes, Quebec, analysis of varve exposures near both the Twin Falls and Frederick House dams, Ontario, and review of published and unpublished logs of varve exposures, reveal that mass transport deposits formed in, or are inferred to have formed in, varve year (vyr) 1483 (relative to the Timiskaming varve series, and about 9.1 ka cal BP) are present at numerous locations within the study area. The spatial extent of the vyr 1483 mass transport deposit signature covers at least 14,600 km2, and is best explained by shaking from a paleoearthquake. The magnitude of the event is estimated to be a minimum of M ∼7.3, which is indicative of a strong paleoearthquake. The vyr 1483 paleoearthquake is one of 11 events of ≥M ∼5, occurring within a 425 varve year period between ∼9.4 and 8.95 ka cal BP, and is likely part of a period of elevated, early postglacial, seismicity associated with rapid crustal unloading. The absence of an equivalent signature within the younger portion of the glaciolacustrine or lacustrine deposits in local lakes, implies that this paleoearthquake is the strongest to occur within the study area since about 9.1 ka cal BP. The highly-precise correlation of widely-spaced mass transport and disturbed deposits provided by varves is essential to the paleoearthquake interpretation.

A local clay-varve chronology of Onega Ice Lake, NW Russia

Onega Ice Lake developed in front of the receding Late Weichselian ice margin in Russian Karelia. Glacial varves that formed in Onega Ice Lake have been earlier studied by means of varve counts, palaeomagnetism and 14C AMS dates from small lakes north of modern Lake Onega and from Lake Onega proper. Synchronous changes in magnetic parameters and similar stratigraphy of these varve records together with the existence of basin-wide marker interval of pink-coloured varves have been used for core-to-core correlation and palaeogeographic interpretations. Unfortunately, there are missing varve-to-varve correlations between these cores. We present a 1155 yr long local varve chronology based on 3 parallel overlapping cores from two small lakes in the Zaonezhsky Peninsula at the northern coast of Lake Onega. Varve counts and matching of varve series were made from digital images. Following the textural, structural, colour and thickness changes of varves within Onega Ice Lake varve series, six characteristic varve units are identified. These clay units reflect changing sedimentary conditions at the time of accumulation that in general can be interpreted as a progressively increasing distance to the retreating ice margin. Variations in varve thickness superimposed on that overall trend, namely two episodes of abrupt and consistent decrease in varve thickness, do not correspond to known lake-level changes or shifts in outflow directions. AMS chronology for deglaciation of Lake Onega basin gives some evidences for correlation of above intervals of decreased varve thickness with the GI-1c2 and GI-1B cold events in NGRIP δ 18O event stratigraphy but further age estimations are needed to confirm presented correlations. Interval of reddish-brown varves – earlier described also as the ‘pink horizon’ – was observed in all studied sequences, and according to varve correlation, at the same stratigraphical level, which proves its basin-wide synchronous appearance. Ca 100 km south of our study area, Saarnisto and Saarinen (2001) reported ca 200 more varves compared to our results. Visual correlation of varve thickness graphs places these extra varves to the proximal part of the series and indicate a rapid ice recession from the main lake basin to the northern Lake Onega area with the ice recession rate ca 500 m per year.

Deglacial record of palaeoearthquakes interpreted from mass transport deposits at three lakes near Rouyn‐Noranda, north‐western Quebec, Canada

AbstractAs part of an investigation of deglacial palaeoseismology within an intracratonic region of north‐eastern North America, the compilation of sub‐bottom acoustic profile surveys from study areas at Duparquet and Dufresnoy lakes, north‐western Quebec, Canada, produced thirteen and eight event horizon maps, respectively, depicting subaqueous mass transport deposits that are interbedded within Lake Ojibway glaciolacustrine varve deposits. Analysis of rhythmic couplets recovered in cores from both locations yielded varve ages with annual resolution for the event horizons relative to the regional Timiskaming varve series. These results are integrated with the published event horizon stratigraphy and varve chronology from nearby Lac Dasserat. The combined event horizon records for the three lakes overlap between varve years 1175 to 1625 (9395 to 8945 ± 200 cal year bp) and consist of 26 event horizons. Fifteen event horizons are inferred to be evidence of 11 palaeoearthquakes at qualitative levels of interpretative confidence varying from high to low. The palaeoearthquake interpretations and assessments of interpretative confidence are made from weightings of the event horizon signatures and, where possible, evidence supporting a stratigraphic footprint extending beyond a given study area. Occurring over a 450 varve year period when rapid crustal uplift was still underway, 11 interpreted palaeoearthquakes happened at an average rate of every ca 40 years. Relative to a published minimum threshold magnitude for generating a recognizable geomorphic signature, this average rate of occurrence exceeds the modern return period by two orders of magnitude. The palaeoearthquakes probably occurred within a period of elevated seismicity associated with rapid crustal unloading during deglaciation.

A new varve thickness record from Allt Bhraic Achaidh Fan, middle Glen Roy, Lochaber: implications for understanding the Loch Lomond Stadial glaciolacustrine varve sedimentation trends

Glen Roy, Lochaber is a key UK site for understanding Late Devensian environmental change, as it contains an annually-resolved glaciolacustrine varve record. This paper develops our understanding of varve sedimentation within Glen Roy through the examination of a new varve sequence located in a more proximal position on the Allt Bhraic Achaidh Fan, one of a series of major fans within the valley. This new varve record consists of c. 203 annual layers, much fewer years than at other sites in the Lochaber area probably due to five significant hiatuses within the record. Varve sediment characteristics and thickness are comparable to, but not statistically correlated with, other varve series that were used to construct a consolidated varve record for the area, the Lochaber Master Varve Chronology. Sedimentological characteristics, analysed by thin section micromorphology, suggest that varve thickness changes within the basin are controlled mainly by distance from the valley sides rather than the position of the ice margin during the Loch Lomond Readvance, as previously proposed.

Evidence of late glacial paleoseismicity from submarine landslide deposits within Lac Dasserat, northwestern Quebec, Canada

An integrated seismo- and chronostratigraphic investigation at Lac Dasserat, northwestern Quebec, identified 74 separate failures within eight event horizons. Horizons E and B, and H and G have strong or moderately-strong multi-landslide signatures, respectively, composed of 11–23 failures, while horizons F, D, C, and A have minor landslide signatures consisting of a single or pair of deposit(s). Cores collected at six sites recovered glacial Lake Ojibway varve deposits that are interbedded with the event horizons. The correlation of the varves to the regional Timiskaming varve series allowed varve ages or ranges of varve ages to be determined for the event horizons. Horizons H, G, E, and B are interpreted to be evidence of paleoearthquakes with differing levels of interpretative confidence, based on the relative strength of the multi-landslide signatures, the correlation to other disturbed deposits of similar age in the region, and the lack or possibility of alternative aseismic mechanisms. The four interpreted paleoearthquakes occurred between 9770 ± 200 and 8470 ± 200 cal yr BP, when glacial Lake Ojibway was impounded behind the Laurentide Ice Sheet during deglaciation. They probably represent an elevated period of seismicity at deglaciation that was driven by crustal unloading.

Lateglacial cryptotephra detected within clay varves in Östergötland, south‐east Sweden

ABSTRACTHere we present a 710‐year‐long floating varve record from south‐east Sweden. Tephra analyses confirm the presence of the rhyolitic Vedde Ash preserved within two consecutive varve years, confirming the Younger Dryas age of the varve series. This permits, for the first time, direct correlation of Swedish varved clay with other records of equivalent resolution which also preserve the Vedde Ash and demonstrates that the potential exists to independently date the Swedish Timescale. This discovery will allow direct comparison of rates, timing and duration of key climatic events across Europe and the North Atlantic region in records of equivalent resolution.

Varved glaciomarine clay in central Sweden before and after the Baltic Ice Lake drainage: a further clue to the drainage events at Mt Billingen

Glaciomarine clay found west of Mt Billingen, central Sweden, contains two distinct varve units separated by a sand layer that we interpret to be sediment deposited in the North Sea during the catastrophic drainage of the Baltic Ice Lake (BIL). The lower varve series was deposited proximal to the retreating ice margin and consists of varves that grade upward from gray to red. The upper varve series was deposited after the drainage event when fresh water within the Baltic basin flowed westward north of Billingen; the upper varves grade from red to gray. Grain size, elemental composition (determined by X-ray fluorescence (XRF)), iron and organic-carbon content vary within each varve, with values that grade upward through each varve, but with a sharp contact with the overlying varve. The two varve units differ from each other, with the lower sequence being coarser and the upper containing a higher iron and organic-carbon content. We attribute the differences between pre- and post-drainage varves to reflect changes in distance to the ice margin and provenance. The stratigraphy at the site suggests that the BIL drainage lasted < 1 year. Lithostratigraphic correlations to the Lake Lången basin west of Mt Billingen support the idea of glacier ice in the Lången basin during the drainage and a retreat and advance of the ice sheet during Alleröd and Younger Dryas, including the possibility of an earlier, Alleröd drainage.

Glacial varves at Pärnu, south-western Estonia: a local varve chronology and proglacial sedimentary environment

A new varve chronology comprising 584 consecutive varve years, varve thickness changes and proglacial sedimentary environment are analysed across the Pandivere–Neva (>13 300 yr BP) belt of ice-recessional formations in coastal Estonia, eastern Baltic. Varved clays in the area have been deposited in the Baltic Ice Lake. All investigated sequences display a normal varve series with decreasing varve thickness upwards. According to the varve correlation, two groups of varve graphs could be distinguished: those representing sequences from the distal side and from the proximal side of the Pandivere–Neva ice marginal formations. Accumulation of varved clays started ca. 100 yr earlier in the distal part. A massive clay unit with corresponding 20 silty varves in some sequences precedes the beginning of varve formation at the proximal part of the basin and could therefore correspond to a ca. 20 yr long ice-stagnation period during the Pandivere–Neva Stade. Ice proximal conditions on both sides of the recessional formations (within 30 km distance) ceased simultaneously within a period of 10–15 yr, which is reflected in reduced total varve thickness accompanied with the beginning of winter layer dominance within a single varve. Because of post-sedimentary erosion, the length of the presented chronology underestimates the duration of proglacial conditions. Good visual correlation between the mean varve thickness graphs from the Pärnu area and from Lake Peipsi, eastern Estonia, indicate the possible regional causes of varve thickness changes and simultaneous varve formation in two basins.

Constrained age of Glacial Lake Narragansett and the deglacial chronology of the Laurentide Ice Sheet in southeastern New England

The lack of radiocarbon ages and correlated varve sequences in southeastern New England has left the deglacial chronology of the region poorly constrained. A 265-year varve series from Glacial Lake Narragansett was constructed from eight continuous sediment cores collected from the Providence River, Narragansett Bay, Rhode Island. This varve series could not be correlated with either the North American Varve Chronology or other varve sequences from southern New England or southeastern New York. The uncorrelated varve sequences presented here represent the minimum time of deposition within the northern segment of Glacial Lake Narragansett. These sequences, used in conjunction with the calibrated North American Varve Chronology and cosmogenic exposure ages from recessional end moraines, provide minimum (>19,400 cal BP) and maximum (<20,500 cal BP) ages for Glacial Lake Narragansett. Correlations with the updated Greenland (NGRIP and GRIP) ice core records suggest that cold periods associated with moraine formation are 200–250 years older than the cosmogenic exposure ages. Whereas many studies refer to the last glacial maximum occurring from 20,000 to 18,000 cal BP, the constrained age of Glacial Lake Narragansett suggests that at least for the southeastern portion of the Laurentide Ice Sheet, deglaciation was well underway by this time.

Late Weichselian varve archives re-explored to assess proglacial sedimentary chronologies using the principles of tree-ring analysis

Glaciogenic varves are formed annually in proglacial basins and provide detailed geochronological archives about the dynamics of deglaciation. Varve chronologies are constructed from multiple varve series measured at individual localities. Temporal connection between the series is traditionally achieved by visual comparison of varve-thickness variations. This connection method resembles that of dendrochronological cross-dating. By contrast, dendrochronological correlation proceeds through the routine of statistical (along with visual) comparisons between the tree-ring series, providing quantification of the dating results, their veracity and robustness. Here we demonstrate the advantages and convenience of applying tree-ring statistical methods to varve records, using an extensive varve archive originating from southern Finland. The data series of individual varve-thickness diagrams were metamorphosed by detrending, prewhitening and averaging, that is, by the methods adopted from tree-ring science. Subsequently, it was seen that the series having 80 or more varves have a higher potential to be unambiguously cross-dated than shorter series. The varve-thickness diagrams could be categorized into two types, those having regional chronological importance and those showing only subregional (distance &lt; 20 km) correlativity. The varves of former type ought to be regarded as primary constituents of the geochronology. The varves of latter type have low geochronological validity although they may bear local sedimentological importance. The findings have the potential to modernize the science of varve-based geochronologies.

Mechanism of Varve Formation and Paleoenvironmental Research at Lake Bolterskardet, Svalbard, the Arctic

Abstract On the basis of observation of thin sections and 137Cs data, laminations in sediment are interpreted to be varves in Bolterskardet Lake (78°06′ N, 16°01′ E), Svalbard, the Arctic. Varves appear under a petrologic microscope as couplets of dark-silt and light-clay layers. The mechanism of varve formation is surmized as follows: each silt layer is the production of sediment inflow interpreted as mainly derived from snowmelt during summer; each clay layer was deposited in a stillwater environment during an ice-cover period. A light-clay layer provides an important index bed to identify the annual interface. The high accumulation rates, long period of ice cover, and topographically closed basin are probably all critical factors in forming and preserving varves. Varve thickness is known to be controlled mainly by summer temperature. The variation of varve thickness in Lake Bolterskardet can then be used to reconstruct summer temperature. The varve series show that there has been distinct decade-scale variability of summer temperature over the past 150 years. Warm periods occurred in the 1860s, around 1900, the 1930s, 1950s, and 1970s, and in the last 20 years. The varved sediments of Lake Bolterskarde preserve an ideal record for high-resolution paleoclimatic and paleoenvironmental research in this data-sparse area.

Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments

Properties of varved sediments from Lake Logurinn in eastern Iceland and their link to climate and glacial processes of Eyjabakkajokull, an outlet glacier of the Vatnajokull icecap, were examined. A varve chronology, which covers the period AD 1262-2005, was constructed from visual observations, high-resolution images, X-ray density and geochemical properties determined from X-radiography and X-ray fluorescence scanning. Independent dating provided by 137Cs analysis and eight historical tephras verify the varve chronology. The thickness of dark-coloured seasonal laminae, formed mainly of coarser suspended matter from the non-glacial river Grimsa, is positively correlated (r=0.70) with winter precipitation, and our 743-year-long varve series indicates that precipitation was higher and more varied during the later part of the Little Ice Age. Light-coloured laminae thickness, controlled mainly by the amount of finer suspended matter from the glacial river Jokulsa i Fljotsdal, increased significantly during the AD 1972 surge of Eyjabakkajokull. As a consequence of the surge, the ice-dammed Lake Haoldulon formed and recurrently drained and delivered significant amounts of rock flour to Lake Logurinn. Based on these observations, and the recurring cyclic pattern of periods of thicker light-coloured laminae in the sediment record, we suggest that Eyjabakkajokull has surged repeatedly during the past 743 years, but with an increased frequency during the later part of the Little Ice Age.

Climatic implications of annual to decadal resolution stable isotope data from calcite varves of the Piànico interglacial lake record, Southern Alps

Stable oxygen isotope analyses at annual, 2-, 5-, 10- and 20-varve sample resolutions were carried out on two selected varve intervals from the interglacial sediment record of the Piànico palaeolake. These sediments are particularly suitable for ultra-high-resolution isotope analyses on lacustrine endogenic calcite because of the exceptionally well-preserved varve structure. A bias through detrital contamination can be excluded because microscopically controlled sampling enabled selecting detritus-free samples. The studied sediment intervals comprise 352 and 88 continuous varve series formed during periods of rapid climate change at the onset and end of a marked millennial-scale cool interval during the Piànico Interglacial. The most intriguing result is a pronounced short-term oscillation in the bi-annually resolved isotope record superimposed on the general decreasing and increasing δ 18O trends at the climatic transitions that is recorded at lower sample resolution. Spectral analyses of the bi-annual time series reveal periodicities indicating solar and NAO controls on the δ 18O record. Multiple δ 18O measurements from endogenic calcite of individual varves showed variations of up to 0.6‰, thus larger than the observed inter-annual variability and most likely explained by seasonal effects.

Linear and nonlinear alignment of time series with applications to varve chronologies

AbstractThe problem of aligning time series arises naturally in a number of fields, but our investigation is motivated by the problem of extracting a paleoclimatic signal from glacial varve chronologies. Glacial varves are laminated sediments that record annual depositional cycles in certain lakes that are fed by glaciers. Varve thickness is determined by temperature and to a lesser extent by precipitation so that an accurate reconstruction of varve chronologies can be regarded as providing a potential long term proxy for paleoclimate. An important aspect of the varve problem is to align series from different locations. What makes the alignment problem particularly interesting is that in either series, one or more varves may be missing or a single varve might have been mistaken for a pair of varves.Traditionally, alignment problems have been treated using linear methods and concepts such as coherency and cross correlation. Since one of the pairs of varve series we look at is not well described by a linear model, in this paper we also employ nonlinear criteria that are derived from recent work on nonlinear identification and dependence modeling. More precisely, we use nonlinear least squares and measures of dependence such as the Kullback—Leibler measure and the Hellinger distance. Both local and global alignment are considered, and in particular we treat the situation where one or more observations may be missing from either series. Properties of the criteria are given and illustrated by simulation. We display examples where linear methods fail, but nonlinear criteria work. For the real data set of varves at our disposal, it will be seen that the nonlinear criteria perform as well as (or better than) the linear ones for the linearly related series, and that only the nonlinear criteria work for the two series that are not linearly related. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Local clay-varve chronology in the Karlskrona-Hultsfred region, southeast Sweden

A local varve chronology from southeast Sweden between the town of Karlskrona in the province of Blekinge and Hultsfred in the province of Småland is presented. The chronology covers approximately 800 varve-years. The glaciolacustrine varves were deposited in the Baltic Ice Lake during the Late Weichselian deglaciation. The study includes 60 connected and 56 unconnected varve series and shows that the ice recession rates vary between 75–125 m/year and 250–340 m/year in the southern and northern parts of the area, respectively. An abrupt change from thin to thick clay varves was found in the northern part of the area. The change has been correlated with a similar change of silty varves in the bottom bed of one of the delta plains close to the highest shoreline. This change in the meltwater deposition has been hypothetically correlated with the transition between the Older Dryas (GI-1d) and Alleröd (GI-1c) chronozones, or around 13,750 GRIP years.

Climatically relevant periodicities in the thickness of biogenic carbonate varves in Spooensee, Switzerland (9740-6870 calendar yr BP)

Accurate determination of the thicknesses of 2717 biogenic carbonate varves from a Swiss lake (Soppensee), spanning the period from about 9740 to 6870 calendar yr BP, allowed the computation of a power spectrum using classical FFT methods. The presence of pronounced peaks at 40-50 and 20-25 yr agrees with the results of other studies on varve thickness and D14C which have been interpreted as indicating an association between solar forcing and varve thickness. The presence in the Soppensee varve series of an additional peak at 10.9 yr, corresponding to the Schwabe sunspot cycle, lends further support to this hypothesis. Because the Soppensee varves are of biogenic rather than clastic origin, any influence fluctuations in solar irradiance may have had on sedimentation rates is likely to have been exerted via primary production.

Exploratory spectral analysis of hydrological times series

Current methods of estimation of the univariate spectral density are reviewed and some improvements are made. It is suggested that spectral analysis may perhaps be best thought of as another exploratory data analysis (EDA) tool which complements, rather than competes with, the popular ARMA model building approach. A new diagnostic check for ARMA model adequacy based on the nonparametric spectral density is introduced. Additionally, two new algorithms for fast computation of the autoregressive spectral density function are presented. For improving interpretation of results, a new style of plotting the spectral density function is suggested. Exploratory spectral analyses of a number of hydrological time series are performed and some interesting periodicities are suggested for further investigation. The application of spectral analysis to determine the possible existence of long memory in natural time series is discussed with respect to long riverflow, treering and mud varve series. Moreover, a comparison of the estimated spectral densities suggests the ARMA models fitted previously to these datasets adequately describe the low frequency component. Finally, the software and data used in this paper are available by anonymous ftp from fisher.stats.uwo.ca.