biological forcing of hemipelagic sedimentary laminae evidence from odp site 893 santa barbara basin california core observation x radiography and scanning electron microanalysis of distinctly laminated indistinctly laminated and nonlaminated sediments from odp site 893 show that laminated as well as nonlaminated i e massive hemipelagic sediments can form in the absence of bioturbation this view contrasts with conventional sedimentological interpretations equating laminated hemipelagic sediments with anoxic water masses i e absence of bioturbation and nonlaminated sedimentary intervals exclusively with oxygenation episodes that permitted infaunal colonization we conclude that heterogeneities in the texture and or composition of sediment supply are necessary for the production of laminated sediments the absence of hydraulic and biological reworking permits their preservation laminae at site 893 have two prominent components biosilica and terrigenous detritus compositional contrasts between adjacent laminae in x radiographs reflect short term seasonal and subseasonal heterogeneities in sediment flux core intervals with large contrasts in density and or composition between adjacent laminae are termed as having high bimodality hb hb couplets produce distinctly laminated sediments that intergrade vertically with indistinctly laminated and nonlaminated sediments adjacent laminae in indistinctly laminated sediments possess minimal contrasts in bulk density and composition nonlaminated sediments are compositionally and texturally homogeneous many biosiliceous laminae responsible for distinctly laminated hb couplets record mass flocculation and sedimentation of ungrazed diatom frustules formed during discrete bloom events ecological interpretation of diatomite laminae suggests that many were effectively self sedimenting i e the production of gelatinous exudates by phytoplankton facilitated their aggregation and rapid sedimentation the results indicate that lamination style preserves meaningful ocean climate data concerning episodicity heterogeneity and export efficiency of biologically mediated sedimentary flux in upwelling dominated coastal ecosystems

climatically controlled marker layers in santa barbara basin sediments and fine scale core to core correlation varved sediments with annual layering can provide a detailed history of environmental conditions of marginal ocean basins and of lakes to reconstruct this history requires fine scale correlation and correct age assignment in the santa barbara basin a combination of x radiography water content profiles and microfossil stratigraphy yields correlations of the desired resolution x radiography permits identification of individual layers 1 2 mm thick that differ in porosity or pore water content and hence x ray absorption microfossils and sedimentological parameters provide the clues to the presence for el nino years e g in a d 1984 1959 and 1942 varves allowing the tie in to absolute years within this century el nino layers once identified are readily recognized on the radiographs and within the porosity profiles counting of laminae in x radiographs and core to core correlation via water content then allow establishing age scales for other cores a prominent shell layer in the first half of the 19th century is dated at a d 1838 this event macoma i is of supraregional significance

age of the barremian aptian boundary and onset of the cretaceous normal superchron abstract the age assigned to the boundary of the barremian and aptian stages remains one of the most poorly constrained post pangean stratigraphic boundaries the lack of a global boundary stratotype section and point gssp for the stage boundary has hampered efforts to calibrate the absolute age of the cretaceous period in the geological time scale the current definition of the barremian aptian boundary also approximates the onset of magnetic polarity chron m0r the end of this chron denotes the start of the cretaceous normal polarity superchron that is of fundamental importance for plate reconstructions currently there is up to 5 discrepancy in the age estimates of the barremian aptian boundary ca 126 121 ma and the start of the cretaceous normal superchron here we review available geochronological information from the late barremian and early aptian stages collected from the pacific ocean china california the ontong java nui large igneous province and the high arctic large igneous province by utilizing only robust geochronological data including u pb and recalibrated 40ar 39ar ages from sites with magnetic polarity information and or paleontological constraints we calculate a best estimate of between 123 8 and 121 8 ma for the barremian aptian boundary and the onset of chron m0r at 2 confidence using estimates of the duration of chron m0r 0 49 0 10 myr 2 we conservatively compute the start of the cretaceous normal superchron to between 123 4 and 121 2 ma 2 using an age of 83 07 0 15 ma 2 for the end of the cretaceous normal superchron the duration of the superchron is also constrained to between 38 0 and 40 5 myr 2 these age ranges for the barremian aptian boundary the onset of the cretaceous normal superchron and the duration of the superchron currently provide the best estimates until a gssp is formally ratified

human responses to middle holocene climate change on california s channel islands abstract high resolution archaeological and paleoenvironmental records from california s channel islands provide a unique opportunity to examine potential relationships between climatically induced environmental changes and prehistoric human behavioral responses available climate records in western north america 7 3 8 ka indicate a severe dry interval between 6 3 and 4 8 ka embedded within a generally warm and dry middle holocene very dry conditions in western north america between 6 3 and 4 8 ka correlate with cold to moderate sea surface temperatures sst along the southern california coast evident in ocean drilling program odp core 893a b santa barbara basin an episode of inferred high marine productivity between 6 3 and 5 8 ka corresponds with the coldest estimated ssts of the middle holocene otherwise marked by warm low productivity marine conditions 7 5 3 8 ka the impact of this severe aridity on humans was different between the northern and southern channel islands apparently related to degree of island isolation size and productivity of islands relative to population fresh water availability and on going social relationships between island and continental populations northern channel islanders seem to have been largely unaffected by this severe arid phase in contrast cultural changes on the southern channel islands were likely influenced by the climatically induced environmental changes we suggest that productive marine conditions coupled with a dry terrestrial climate between 6 3 and 5 8 ka stimulated early village development and intensified fishing on the more remote southern islands contact with people on the adjacent southern california coast increased during this time with increased participation in a down the line trade network extending into the western great basin and central oregon genetic similarities between middle holocene burial populations on the southern channel islands and modern california uto aztecan populations suggest middle holocene movement of people at this time from southern california desert environs westward to the southern islands a migration perhaps stimulated by increased continental aridity

tectonic and lithologic controls on bedrock channel profiles and processes in coastal california 1 recent theoretical models suggest that topographic characteristics of bedrock channels are products of interactions among tectonics substrate resistance and the climatically modulated erosive ability of the river the degree to which these factors influence the form of channel profiles is poorly quantified at present here we investigate bedrock channels developed across the southern flank of the santa ynez mountains california uniform climate and systematic variations in lithology and rock uplift rate along the range allow comparison of channel morphology between 1 channels eroding rocks of uniform and nonuniform strength and 2 channels experiencing differences in tectonic forcing we combine field observations surveys and analysis of digital data to determine topographic and hydraulic characteristics of bedrock channels at a constant rock uplift rate streams flowing from resistant to less resistant bedrock exhibit highly concave profiles and increased gradients along lower reaches relative to channels developed in uniform bedrock these effects are interpreted as responses to 1 an increase in substrate resistance to channel incision in the upper reaches and 2 transport limited gradients along lower reaches comparisons of channels developed across uniform lithology but experiencing an approximately sevenfold difference in rock uplift rate reveal an approximately twofold increase in gradient and an approximately threefold decrease in width in this landscape the combined channel adjustments of gradient and width are consistent with a fluvial incision model in which channel incision rate is linearly proportional to mean bed shear stress

global boundary stratotype section and point gssp for the anthropocene series where and how to look for potential candidates abstract the anthropocene as a potential new unit of the international chronostratigraphic chart which serves as the basis of the geological time scale is assessed in terms of the stratigraphic markers and approximate boundary levels available to define the base of the unit the task of assessing and selecting potential global boundary stratotype section and point gssp candidate sections a required part of the process in seeking formalisation of the term is now being actively pursued here we review the suitability of different stratified palaeoenvironmental settings and facies as potential hosts for a candidate gssp and auxiliary sections and the relevant stratigraphical markers for correlation published examples are evaluated for their strengths and weaknesses in this respect a marked upturn in abundance of radioisotopes of 239pu or 14c approximately in 1952 and 1954 ce respectively broadly coincident with a downturn in 13c values is applicable across most environments principal palaeoenvironments examined include settings associated with accumulations of anthropogenic material marine anoxic basins coral reefs estuaries and deltas lakes at various latitudes peat bogs snow ice layers speleothems and trees together many of these geographically diverse palaeoenvironments offer annual subannual laminae that can be counted and independently dated radiometrically e g by 210pb examples of possible sections offer the possibility of correlation with annual seasonal resolution from among such examples a small number of potentially representative sites require the acquisition of more systematic and comprehensive datasets with correlation established between sections to allow selection of a candidate gssp and auxiliary stratotypes the assessments in this paper will help find the optimal locations for these sections

climate change and the episodicity of sediment flux of small california rivers abstract we studied the streamflow and sediment flux characteristics of the 20 largest streams entering the pacific ocean along the central and southern california coast extending for 750 km from monterey bay to just south of the u s mexico border drainage basins ranged in area from 120 to 10 800 km2 with headwater elevations ranging from 460 to 3770 m annual streamflow ranged from 0 to a maximum of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 1 times 10 9 end document m3 yr for the santa clara river in 1969 with a

implications of radiocarbon ages of organic and inorganic carbon in coastal lakes in florida for establishing a reliable chronology for sediment based paleoclimate reconstruction coastal lake sediments are valuable paleoclimate archives provided that they can be accurately dated here we report radiocarbon ages of bulk sediment organic matter om plants shells particulate om and dissolved om from coastal lakes in florida bulk sediment om yielded ages that are consistently older than contemporaneous plants and shells indicating significant radiocarbon deficiencies in sedimentary om in these lakes the data show that the om radiocarbon deficiency varies over time and with location making it impossible to determine a proper correction factor for radiocarbon ages of bulk sediments from these lakes as a result we consider ages obtained from bulk sediment om from these lakes unreliable the age reversals in bulk sediment om observed in the sediment cores are likely caused by rapid increases in erosion and sedimentation resulting from large storm events the data also show that sedimentation rate can vary considerably within a given lake implying that an age depth model established for one core cannot be directly applied to other cores despite their close proximity analyses of shells from one of the lakes suggest that fresh brackish water shells may serve as a good substrate for radiocarbon dating owing to a small reservoir effect on inorganic carbon

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation

historical oceanographic events reflected in13c 12c ratio of total organic carbon in laminated santa barbara basin sediment an 1844 1987 time series of carbon stable isotope ratios from dated sedimentary total organic carbon toc from the center of the santa barbara basin sbb is compared with historical climate and oceanographic records four isotopically distinct biogeochemical sources of toc are important phytoplankton derived marine biomass macroalgal biomass from kelp forests terrigenous biomass mainly flushed into the sbb via river discharge and redeposited fossil organic carbon the significance of the latter two sources is largely limited to a few unusual flood and oil spill events whereas the combination of 13c depleted phytoplankton and 13c enriched macroalgal biomass appears to be responsible for most of the isotopic variance of the marine coastal biomass as recorded in sedimentary toc the isotopic response of marine organic carbon in sediments records strong el nino southern oscillation enso and the frequently associated severe storm and wave events in sbb varved sediments the plausible major isotopic mechanisms are 1 increased physical liberation of 13c enriched kelp carbon from locally abundant giant kelp macrocystis spp forests during times of physical and environmental stress and 2 decreased productivity of 13c depleted phytoplankton during enso events

vegetation response to southern california drought during the medieval climate anomaly and early little ice age ad 800 1600 abstract high resolution studies of pollen in laminated sediments deposited in santa barbara basin sbb core spr0901 02kc reflect decadal scale fluctuations in precipitation spanning the interval from ad 800 1600 from ad 800 1090 during the medieval climate anomaly mca sbb sediments were dominated by xeric vegetation types drought resistant coastal sagebrush and chaparral implying reduced precipitation in the southern california region drought adapted vegetation abruptly decreased at ad 1090 and was rapidly replaced by mesic oak quercus woodlands associated with an increased pollen flux into the basin after a mesic interval lasting 100 years pollen flux and the relative abundance of quercus pollen dropped abruptly at ad 1200 when the rapid rise of chaparral suggests a significant drought similar to that of the mca ad 800 1090 this brief resurgence of drought adapted vegetation between ad 1200 1270 marked the end of the mca droughts a gradual increase in mesic vegetation followed characterizing cool hydroclimates of the little ice age lia in coastal southern california the presence of xeric vegetation in sbb coincides with major drought events recorded in tree rings and low lake levels elsewhere in california except for the brief drought between ad 1130 1160 correlative diatom and terrigenous sediment input proxy records from sbb are largely supportive of the pollen record predominantly linking the mca with drought and la nina like conditions and the lia with wetter more el nino like conditions differences between paleoclimate proxies pollen diatoms and terrigenous sediment in sbb exist however possibly reflecting the temporal and spatial differences in the generation of each proxy record as well as their individual sensitivity to climate change

a holocene record of ocean productivity and upwelling from the northern california continental slope abstract the holocene upwelling history of the northern california continental slope is examined using the high resolution record of tn062 o550 40 9 n 124 6 w 550 m water depth this 7 m long marine sediment core spans the last 7500 years and we use it to test the hypothesis that marine productivity in the california current system ccs driven by coastal upwelling has co varied with holocene millennial scale warm intervals a combination of biogenic sediment concentrations opal total organic c and total n stable isotopes organic matter 13c and bulk sedimentary 15n and key microfossil indicators of upwelling were used to test this hypothesis the record of biogenic accumulation in tn062 o550 shows considerable holocene variability despite being located within 50 km of the mouth of the eel river which is one of the largest sources of terrigenous sediment to the northeast pacific ocean margin a key time interval beginning at 2900 calibrated years before present cal yr bp indicates the onset of modern upwelling in the ccs and this period also corresponds to the most intense period of upwelling in the last 7500 years when these results are placed into a regional ccs context during the holocene it was found that the timing of upwelling intensification at tn062 o550 corresponds closely to that seen at nearby odp site 1019 as well as in the santa barbara basin of southern california other ccs records with less refined age control show similar results which suggest late holocene upwelling intensification may be synchronous throughout the ccs based on the strong correspondence between the alkenone sea surface temperature record at odp site 1019 and the onset of late holocene upwelling in northern california we suggest that ccs warming may be conducive to upwelling intensification though future changes are unclear as the mechanisms forcing sst variability may differ

evidence for temporal fluctuations in marine radiocarbon reservoir ages in the santa barbara channel southern california abstract differences in the 14 c ages of closely associated marine shell and carbonized plant material from stratified archaeological deposits on san miguel island california suggest holocene 10 000 present fluctuations in marine 14 c reservoir ages these fluctuations coincide with 18 o and 13 c shifts measured in mytilus californianus shells from the same stratigraphic contexts and general atmospheric oceanic circulation models for the region based on these data we make three primary observations 1 significant changes appear to have occurred in the radiocarbon reservoir during the holocene 2 these fluctuations appear to correlate with regional oceanographic changes and 3 high resolution 14 c dating of marine shells may require different r values for different periods of time

a 900 year stable isotope record of interdecadal and centennial change from the california current this paper describes the development of a high resolution record of interdecadal and centennial variability of ocean climate of the california current from continuous 5 year sampling intervals based on varve counts from santa barbara basin sediments we show the importance of the choice of species and size fraction as well as the practice of making multiple measurements per interval to obtain a clear environmental signal capable of resolving interdecadal change changes in temperature and salinity from oceanographic surveys of the past 50 years indicate that thermal variability dominates the 18o signal a 18o record from neogloboquadrina dutertrei spanning 1030 1905 a d contains interdecadal thermal fluctuations within the upper thermocline that are centered around periods of roughly 55 70 and 90 years the interdecadal changes comprise the major share of total variability compared to a low amplitude mode of underlying multicentennial variability corresponding to the medieval warm period and the little ice age

the early cretaceous barents sea sill complex distribution 40ar 39ar geochronology and implications for carbon gas formation abstract mafic igneous rocks of cretaceous age 80 130 ma scattered around the arctic ocean are commonly referred to as the high arctic large igneous province halip we have mapped out the distribution of halip igneous rocks in the barents sea region over the past decade based on integrated seismic gravity magnetic interpretation field work review of publications and analyses of new and vintage borehole and field samples the mapping reveals abundant igneous rocks in the northern and eastern barents sea covering an area of 900 000 km2 with a conservative volume estimate of 100 000 to 200 000 km3 of intrusions the igneous province is dominated by sheet intrusions injected into triassic and permian sedimentary rocks hydrothermal vent complexes are rare and only two potential vent complexes have been identified on seismic data in the eastern barents sea we have further done extensive radiometric dating of the igneous samples in the barents sea region new 40ar 39ar dating of thirteen samples from svalbard reveal ages of crystallization and alteration the large age span 60 140 ma for the raw ages is likely due to partial or complete overprint of the k ar system in plagioclase and the age of the magma emplacement is better represented by u pb tims ages only one of our 40ar 39ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published u pb tims ages of 122 125 ma the new data clearly document that relying on published data from the k ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40ar 39ar degassing data i e k ca we propose that the magmatism on svalbard and franz josef land represents a distinct magmatic event near the barremian aptian boundary 125 ma in the barents sea this early cretaceous barents sea magmatism resulted in the formation of the bssc barents sea sill complex bssc age rocks are also present in arctic canada sverdrup basin and on bennett island new siberia islands the massive injection of hot magma into potentially organic rich sediments in the eastern and northern barents basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles we estimate that up to 20 000 gt of carbon were potentially mobilized corresponding to 175 trillion barrels of oil equivalent the production rates and fate of the carbon gases are uncertain however we speculate that rapid release of aureole greenhouse gases methane may have triggered the oceanic anoxic event 1a oae1a and the associated negative 13c excursion in the early aptian some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east barents basin

subtropical hydroclimate during termination v 430 422 ka annual records of extreme precipitation drought and interannual variability from santa barbara basin abstract hydroclimate extremes are expected to become more frequent and intense with anthropogenic climate forcing but future el nino southern oscillation enso behavior is unclear understanding of extreme hydroclimate variability and magnitude prior to human influences is limited by short instrumental records however continuous sedimentary archives of past hydroclimate variability can complement and extend these records laminated santa barbara basin sbb sediment cores mv0508 33jpc 21jpc and 29jpc preserve annually resolved multi centennial scale precipitation records from termination v t v the transition from glacial marine isotope stage mis 12 to interglacial mis 11 at 424 ka these records provide insights into the subtropical hydroclimate response to warming foraminiferal 18 o variations indicate rapid warming and or salinity changes and were used to verify the t v age assignment a paleoprecipitation proxy was developed using the first principal component of scanning xrf elemental counts pc1 which has high loadings for siliciclastic sediment associated elements k ti and si sedimentary laminae couplets identified in pc1 were annually tuned to investigate t v paleoprecipitation variability as modern sbb laminae represent annual deposits extreme flooding and decadal to centennial droughts were identified with magnitudes exceeding modern observations enso like 2 7 year paleoprecipitation periodicities coincide with wetter intervals but enso variability is reduced during droughts a 1500 year arid interval may be related to poleward shifting of general atmospheric circulation as ice sheets melted such that the subtropical dry zone intersected california southern california paleoprecipitation reconstructions from past warm climates provide insight into precipitation variability and magnitude on interannual timescales and can reduce uncertainties in predictions of interannual climate including enso

brief interstadial events in the santa barbara basin ne pacific during the past 60 kyr the instability of the northern hemisphere glacial climate over the past 100 kyr has been revealed by at least 20 brief warm interstadial episodes called dansgaard oeschger events recorded in greenland ice cores1 3 and in north atlantic sedimentary records4 5 a few of these events have been recognized elsewhere6 8 here we describe a record of ocean oxygenation and circulation from the santa barbara basin in the northeast pacific ocean which correlates well with the greenland ice core records we see 19 of the 20 dansgaard oeschger events in the form of laminated sediments deposited under anoxic conditions and we can correlate at least 16 of these with the 17 ice core interstadials of the past 60 kyr thus these short term events were not restricted to the north atlantic region the events had substantial ecological and oceanographic effects in the santa barbara basin including changes in benthic faunal populations and in the age and composition of bottom waters similar ventilation changes have been seen in the gulf of california9 10 suggesting that these changes may have been widespread and synchronous along the northeast pacific margin these results suggest sensitivity of broad areas of the ocean atmosphere cryosphere system to short term climate change

sediment yield from the tectonically active semiarid western transverse ranges of california sediment yields from the world s rivers are generally highest from steep drainage basins with weak lithology active tectonics or severe land use impacts here we evaluate sediment yields from the western transverse ranges of california in an attempt to explain why they are two to tenfold greater than the surrounding areas of california we found that suspended sediment yields across the gauged basins of the western transverse range during 1969 1999 varied by approximately an order of magnitude 740 5300 t km2 yr similarly fine sediment concentrations for normalized discharge rates varied by almost two orders of magnitude e g 1 3 110 g l for the mean annual flood for 11 previously unmonitored drainages of the santa ynez mountains areas with high sediment yields consistently have weakly consolidated bedrock quaternary pliocene marine formations and are associated with the highest rates of tectonic uplift of the region 5 mm yr these regions are important to the sediment discharge budgets because 50 of the total suspended sediment discharge from the western transverse range is estimated to be generated within these regions even though they represent only 10 of the total watershed area previous estimates of suspended sediment discharge from the ventura river have likely been underestimated by 50 because the gauging station is located immediately upstream of a high sediment yield region we also found a significant and positive correlation between sediment yield and the percentage of a watershed with grassland and agricultural land use these results suggest that there is adequate variation within the lithology tectonics and land use of the broader western transverse range geologic province to induce large variations in sediment yield at the local scale

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

the flushing of two deep sea basins southern california borderland a time series of water temperature salinity and nutrient measurements has been made in san pedro and san nicolas basins of the southern california borderland between i977 and 1988 these basins exhibit temporally coincident changes in temperature and nutrient content defining a period of basin flushing 1982 1984 and a period of stagnation 1984 1987 during the flushing period cooler oxygenand no rich water replaces warmer oxygenand no depleted bottom water flushing produces a decrease of 0 2 c in the average temperature of san pedro basin bottom water 790 900 m and an increase in the average oxygen content by 12 pm these values are 0 i and 3 for san nicolas basin 1 400 l 800 m the residence time of water in san pedro basin during the flushing period is 12 6 months and j t2 months in san nicolas basin a box model approach constrained by the time series data for the stagnation period was used to determine values of the effective vertical eddy diffusivities for the 790 m horizon in san pedro basin and 1 400 m horizon in san nicolas basin 1 3 0 4 and 9 9k3 4 cm sk respectively

increased northeast pacific climatic variability during the warm middle holocene 1 we use a pair of decadally resolved planktonic foraminiferal 18o records from santa barbara basin sbb to show that the warmest interval of the current interglacial the middle holocene was characterized by increased decadal to centennial scale climatic variability modern relationships between the pacific decadal oscillation pdo the el nino southern oscillation enso and upper water conditions in sbb also suggest a preference of the northeast ne pacific climate during the warm mid holocene toward the warm phase of the pdo and more intense enso warm events el ninos in the context of future climate this study suggests increasing decadal to centennial scale climatic variability in a globally warmed world

non annual laminations and expansion of anoxic basin floor conditions in santa monica basin california borderland over the past four centuries sediments from box cores in the deep portion of santa monica basin have been examined by x radiography and the upper 10 20 cm are characterized by laminations 1 5 mm in thickness age dating of 9 cores by 210pb analysis has shown that accumulation rates on the central basin floor have been remarkably constant during the past century averaging 16 0 0 4 mg cm2 yr densitometer analysis of the x radiography films from 7 cores and spectral analysis of the resulting time series indicates that most lamination couplets have a periodicity in the 3 7 year range a range matching that of the el nino southern oscillation periodicity the median periodicity appears to have decreased slightly during the past 3 centuries from 7 0 years during 200 300 yrs b p to 5 2 yrs during 0 100 yrs b p this trend could be due to a decreasing sedimentation rate an increasing forcing frequency or an increasingly faithful response of the basin in more recent years multiple processes have contributed to creation of the laminations in santa monica basin the principal factors appear to be 1 variations in porosity due to the relative abundance of organic filaments probably formed by beggiatoa mats and 2 compositional variations that reflect variations in carbonate lithogenous ratios and in grain size both of these are expected to respond to enso events the presence of near anoxic bottom waters has prevented bioturbation permitting laminations to be preserved basin bottom waters became nearly anoxic about 350 years ago at the onset of warming as the little ice age waned expansion of the near anoxic area during the past century may have been augmented by 20th century anthropogenic effects the style of these non annual laminations in santa monica basin matches that of the annual laminations in the adjacent santa barbara basin and demonstrates the need for caution in interpreting all laminations as representing annual varves

resources for paleoceanographic and paleoclimatic analysis a 6 700 year stratigraphy and regional radiocarbon reservoir age δr record based on varve counting and 14c ams dating for the santa barbara basin offshore california u s a abstract holocene sediments from the central santa barbara basin sbb contain invaluable paleoceanographic information and document paleoclimatic supra regional teleconnections the well constrained dating and stratigraphy of the sbb can serve as a model for the paleoclimatic exploitation of laminated sediments elsewhere here we present an improved 6 700 yr master chronology that is based on sequential downcore counting of annual varves and 14c ams dating starting from the june 2004 sediment surface diagnostic lamination patterns and the presence of prominent marker layers in an 9 m long sequence of x radiographic images from various cores including odp hole 893a and piston core 6p offer convenient dating for other central sbb sediment cores via direct x radiographic and or visual correlation against the master chronology presented here the new sequence of x radiographs of piston core 6p spans the coring hiatus at 7 04 mbsf in odp hole 893a varve count ages agree well with an independent chronology based on calibrated 14c ages of planktic foraminifers in contrast 14c ages of benthic foraminifers and of kerogen from flood layers are more strongly affected by variable 14c reservoir effects and are therefore of limited value for absolute dating of sbb sediment layers a late holocene shift towards larger 14c age differences between planktic and benthic foraminifers after 2 000 yr b p suggests influx of a more 14c depleted thus older and oxygen depleted source of sub sill bottom water into the sbb consistent with the absence of bioturbating benthos and the enhanced preservation of varves observed in the cores the independent varve chronology allowed us to derive a record of the regional radiocarbon reservoir age r by comparing with the 14c dates for the last 6 700 years

coastal upwelling in the california current system abstract coastal upwelling in the california current system has been the subject of large scale studies off california and baja california and of small scale studies off oregon recent studies of the winds along the entire coast from 25 n to 50 n indicate that there are significant along shore variations in the strength of coastal upwelling which are reflected in the observed temperature distribution active upwelling appears to be restricted to a narrow coastal band about 10 25 km wide along the entire coast but the region influenced by coastal upwelling may be much wider intensive observations of the upwelling zone during summer off oregon show the presence of a southward coastal jet at the surface a mean vertical shear a poleward undercurrent along the bottom and persistently sloping isopycnals over the continental shelf most of the upwelling there occurs during relatively short periods several days long of upwelling favorable winds during the upwelling season off oregon the offshore ekman transport is carried by the surface ekman layer and the onshore return flow occurs through a quasi geostrophic interior it is not known whether the structure and dynamics observed off oregon are typical of the upwelling zone along the entire coast though some of the same features have been observed off baja california current and future research will eventually show whether the oregon results are also applicable in the region of persistently strong upwelling favorable winds off northern california and in the region of complex bathymetry off central and southern california

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

abrupt termination of marine isotope stage 16 termination vii at 631 5 ka in santa barbara basin california the marine isotope stage 16 15 boundary termination vii is the first deglacial warming step of the late quaternary following the mid pleistocene transition mpt when 41 kyr climatic cycles shifted to strong 100 kyr cycles the detailed structure of this important climatic event has remained unknown until now core mv0508 19jpc from santa barbara basin california contains a decadal scale climatic and geochemical sediment record of 4000 years duration that includes the early part of this deglacial episode this record reveals that the climatic shift during the early deglacial occurred rapidly 700 years in a progression of three abrupt warming steps the onset of marine isotope stage mis 15 was remarkably abrupt with 4 5 c sea surface warming in 50 years the deglacial sequence contains the well dated lava creek tephra 631 3 4 ka from yellowstone caldera used to date the onset of termination vii at 631 5 ka the late mis 16 and early mis 15 interval exhibits multiple decadal scale negative excursions in 13c of planktic foraminifera likely the result of repeated discharges of methane from methane hydrates associated with both ocean warming and low sea level a warm interstadial that interrupts late mis 16 is marked by elevated concentrations of redox sensitive elements indicating sulfidic oxygen deficient bottom and pore waters and elevated concentrations of total organic carbon and cd reflecting increased surface productivity unlike younger sediments on the california margin these indicators of increased productivity and low dissolved oxygen do not consistently correspond with each other or with preserved laminations possibly reflecting instability of a still evolving ocean atmosphere system following the mpt

expansion of the southern california oxygen minimum zone during the early to mid holocene due to reduced ventilation of the northeast pacific abstract oceanic deoxygenation observed since the 1950s has drawn increasing concern as dissolved oxygen do regulates marine biodiversity and biogeochemical cycles yet the do time series from direct instrumental measurements is insufficiently long several decades to constrain the centennial to millennial variability here we present redox sensitive metal re u and mo records from santa barbara basin sbb california to reconstruct subsurface do through the holocene significantly higher authigenic metal fluxes and coherent benthic foraminifera faunal responses along a published depth transect reveal vertical expansion and intensification of the southern california oxygen minimum zone omz prior to 6 ka comparison with post industrial trace metal records suggests low oxygen conditions were more extreme during early to mid holocene mh highlighting the magnitude of omz variability in the absence of anthropogenic influences inconsistencies between authigenic metal fluxes and local oxygen control indicate a remote weakening of the north pacific intermediate water npiw formation prior to the mh reduced ventilation of the mid depth north pacific is supported by state of the art earth system model simulations with realistic mh 6 ka forcing highlighting the importance of brine rejection during sea ice formation in changing the buoyancy of npiw and its basin wide influences on north pacific omz evidence from holocene marine lake records and model simulations further suggests a southward intertropical convergence zone itcz migration and a consequent deepening of the aleutian low al towards late holocene which favors more sea ice brine rejection thus mid depth ventilation in the north pacific was closely associated with atmospheric circulation during the holocene

biotic response to late quaternary rapid climate switches in santa barbara basin ecological and evolutionary implications benthic foraminiferal assemblages from santa barbara basin exhibit major faunal and ecological switches associated with late quaternary millennial to decadal scale global climate oscillations repeated turnovers of entire faunas occurred rapidly 40 400 yr without extinction or speciation in conjunction with dansgaard oeschger shifts in thermohaline circulation ventilation and climate confirming evolutionary model predictions of roy et al consistent faunal successions of dysoxic taxa during successive interstadials reflect the extreme sensitivity and adaptation of the benthic ecosystem to the rapid environmental changes that marked the late quaternary and possibly other transitional intervals in the history of the earth s ocean atmosphere cryosphere system these data support the hypothesis that broad segments of the biosphere are well adapted to rapid climate change

differences in radiocarbon age between shell and charcoal from a holocene shellmound in northern california abstract the west berkeley shellmound the oldest well dated archaeological site in the san francisco bay region contains shell and charcoal ranging in age from ca 1200 to 5700 cal yr b p radiocarbon ages of marine shell and charcoal collected from fifteen stratigraphic levels in the west berkeley shellmound suggest changes in the 14 c content of san francisco bay surface waters relative to the atmosphere the oceanic reservoir age over the past 5000 yr the reservoir age of san francisco bay waters fluctuated between 870 and 170 14 c yr over the past 5000 yr with the lowest values occurring 2900 to 3800 cal yr b p and the highest values between 1200 and 2000 cal yr b p changes in the radiocarbon reservoir age may be due to changes in the strength of seasonal wind driven upwelling off coastal california where upwelling brings 14 c depleted waters to the surface the period of lowest r values at 3500 to 3900 cal yr b p is coincident with relatively low salinity in san francisco bay indicating high freshwater inflow and wet climate in california based on lake level records the period of high r values 1200 2000 cal yr b p is coincident with one of the driest periods in california during the late holocene these data suggest a link between coastal upwelling and precipitation over central california the age of the top of west berkeley mound and several other mounds in the san francisco bay region 1100 to 1300 cal yr b p coincides with a prolonged dry period in california and low river inflow to san francisco bay perhaps the sites were abandoned because of the drought

california current system response to late holocene climate cooling in southern california 1 new holocene high resolution planktonic foraminiferal assemblage data from santa barbara basin california documents variability in ocean circulation as the california current system responded to millennial scale climate change during late holocene climatic cooling climatic variability increased at 4 ka when a series of extreme cool events notably at 2 2 1 5 and 0 8 ka associated with glacial advance in the pacific northwest punctuated the predominantly warm holocene simultaneously high subtropical species abundance suggests increased interannual variability el nino frequency severity or greater seasonality during the late holocene planktonic foraminiferal assemblages indicate that the magnitudes of climatic shifts were greatest after 1 5 ka during an interval of extreme terrestrial hydrological variability in western north america and that the coolest interval of the holocene in santa barbara basin was coincident with the little ice age

a study of the tex86 paleothermometer in the water column and sediments of the santa barbara basin california 1 particulate organic matter collected during a 2 year period as part of an ongoing sediment trap study and a high resolution sediment record from 1850 to 1987 a d from the santa barbara basin were analyzed for tex86 a temperature proxy based on marine crenarchaeotal membrane lipids highest fluxes of crenarchaeotal lipids in the water column were found in may june 1996 and from october 1996 to january 1997 and in general showed a good correlation with mass fluxes tex86 reconstructed temperatures from the sediment trap series ranged from 8 to 11 c and were usually substantially lower than sea surface temperatures sst indicating that unlike in previous studies the tex86 corresponds to subsurface temperatures likely between 100 and 150 m tex86 temperature variations observed in trap samples were not coupled to changes in sst or deep water temperatures and only to some degree with crenarchaeotal lipid fluxes this suggests that a complex combination of different depth origins and seasonal growth periods of crenarchaeota contributed to the variations in tex86 signal during the annual cycle tex86 temperatures in the two sediment cores studied 8 13 c were also substantially lower than those of instrumental sst records 14 17 5 c confirming that tex86 records a subsurface temperature signal in the santa barbara basin this result highlights the importance of performing calibration studies using sediment traps and core tops before applying the tex86 temperature proxy in a given study area

from extreme rainfall to drought 250 years of annually resolved sediment deposition in santa barbara basin california abstract the mediterranean climate of southern california is marked by droughts and extreme precipitation events here we use elemental variations generated by scanning x ray fluorescence xrf to identify droughts and floods in recently deposited 1755 2008 sediments of santa barbara basin sbb from box core spr0901 04bc the first principal component pc1 of the scanning xrf elements has high loadings for elements associated with the lithogenic component of sbb laminae couplets while the second pc2 is associated with biogenic components we interpret pc1 as a proxy for river runoff and pc2 as a proxy for marine productivity high values of pc1 are associated with el nino events and positive warm phases of the pacific decadal oscillation pdo while low values of pc2 are associated with el nino events and negative cool phases of the pdo droughts such as the 1934 40 1949 56 and 1989 91 events coincide with low pc1 values in addition to distinguishing interannual and decadal variability in the elemental composition of sbb sediments several historic floods can be recognized including a gray flood layer associated with the 1861 62 flood and a peak in pc1 associated with the flood following the st francis dam disaster in 1928

high resolution paleoclimatology of the santa barbara basin during the medieval climate anomaly and early little ice age based on diatom and silicoflagellate assemblages in kasten core spr0901 02kc abstract diatom and silicoflagellate assemblages documented in a high resolution time series spanning 800 to 1600 ad in varved sediment recovered in kasten core spr0901 02kc 34 16 845 n 120 02 332 w water depth 588 m from the santa barbara basin sbb reveal that sbb surface water conditions during the medieval climate anomaly mca and the early part of the little ice age lia were not extreme by modern standards mostly falling within one standard deviation of mean conditions during the pre anthropogenic interval of 1748 1900 no clear differences between the character of mca and the early lia conditions are apparent during intervals of extreme droughts identified by terrigenous proxy scanning xrf analyses diatom and silicoflagellate proxies for coastal upwelling typically exceed one standard deviation above mean values for 1748 1900 supporting the hypothesis that droughts in southern california are associated with cooler or la nina like sea surface temperatures ssts increased percentages of diatoms transported downslope generally coincide with intervals of increased siliciclastic flux to the sbb identified by scanning xrf analyses diatom assemblages suggest only two intervals of the mca at 897 to 922 and 1151 1167 when proxy ssts exceeded one standard deviation above mean values for 1748 to 1900 conversely silicoflagellates imply extreme warm water events only at 830 to 860 early mca and 1360 to 1370 early lia that are not supported by the diatom data silicoflagellates appear to be more suitable for characterizing average climate during the 5 to 11 year long sample intervals studied in the spr0901 02kc core than diatoms probably because diatom relative abundances may be dominated by seasonal blooms of a particular year

coastal flooding and the 1861 2 california storm season abstract a series of large storms attributed to atmospheric river conditions struck the california coast in the winter of 1861 2 although historical accounts document inland flooding little is known about how the 1861 2 storms impacted the now heavily developed california coast here we show that the 1861 2 storms emplaced a deposit of beach sand up to 50 cm thick over 450 m inland within a southern california salt marsh this deposit is unprecedented in the post european sediments of the marsh and more extensive than that derived from any other historical event it is comparable in scale to hurricane and tsunami washover fans in back barrier environments along other coastlines the presence of overwash deposits in carpinteria suggests that the 1861 2 storm season was erosive enough to remove coastal barriers allowing for inundation of parts of the coastline currently developed efforts to prepare for a recurrence of an 1861 2 like storm season should address potential coastal impacts likewise interpretations of past washover deposits should consider these unusually prolonged stormy periods in addition to hurricane and tsunami inundation

a synchronized dating of three greenland ice cores throughout the holocene as part of the effort to create the new greenland ice core chronology 2005 gicc05 a synchronized stratigraphical timescale for the holocene parts of the dye 3 grip and ngrip ice cores is made by using volcanic reference horizons in electri cal conductivity measurements to match the cores the main annual layer counting is carried out on the most suited records only exploit ing that the three ice cores have been drilled at locations with different climatic con ditions and differences in ice flow however supplemental counting on data from all cores has been performed between each set of reference horizons in order to verify the valid ity of the match after the verification the main dating is transferred to all records us ing the volcanic reference horizons as tie points an assessment of the mean annual layer thickness in each core section confirms that the new synchronized dating is consistent for all three cores the data used for the main annual layer counting of the past 7900 years are the dye 3 grip and ngrip stable isotope records as the high accumulation rate at the dye 3 drill site makes the seasonal cycle in the dye 3 stable isotopes very resistant to firn diffusion an effort has been made to extend the dye 3 holocene record the new syn chronized dating relies heavily on this record of 75 000 stable isotope samples the dat ing of the early holocene consists of an already established part of gicc05 for grip and ngrip which has now been transferred to the dye 3 core gicc05 dates the younger dryas termination as defined from deuterium excess to 11 703 b2k 130 years earlier than the previous grip dating

a 9000 year flood history for southern california a revised stratigraphy of varved sediments in santa barbara basin abstract the center of santa barbara basin sbb preserves annual laminations through most of the holocene providing an important locality for high resolution late quaternary paleoclimate and paleoceanography reconstructions an accurate chronology is necessary for these sediment based records and enables comparisons with more distal data time series of similar quality here we present an improved high resolution radiocarbon chronology for the last 9000 yrs based on 89 accelerator mass spectrometric ams 14c dates of mixed planktonic foraminiferal carbonate from three sediment cores collected in sbb mv0811 14jc spr0901 06kc and odp hole 893a accurate core to core correlation is demonstrated using prominent gray flood and olive turbidite layers identified in sbb and dated in multiple cores gray layer deposits were found more frequently in wetter intervals as determined by multi proxy hydroclimate records in southern california e g lake deposits and tree rings mass accumulation rates mars calculated for the sbb depocenter using the improved radiocarbon age model indicate high mars are associated with these gray layers that have been associated with floods thus sediment accumulation in sbb is largely controlled by sediment delivery via river runoff following precipitation events although mars may also be influenced by other factors that enhance erosion in river catchments folded sediment is observed within the basal core section of mv0811 14jc which stratigraphically correlates with 14c age reversals in odp hole 893a we associate this sediment slump feature with the large goleta submarine slide complex in santa barbara channel and estimate the submarine slide event occurred at 9000 200 yrs bp furthermore small olive turbidite layers identified and dated in sbb can be temporally linked with large earthquakes along san andreas fault and can therefore potentially reconstruct earthquake history in southern california

resolving varve and radiocarbon chronology differences during the last 2000 years in the santa barbara basin sedimentary record california abstract santa barbara basin sbb is well known for producing high quality paleoclimate reconstructions but the validity of correlations between the basin and other regions rests upon age model accuracy yet the two independent and well established chronologies based on varve counting and radiocarbon dating do not consistently agree during the last two millennia here the validity of age model assumptions is tested including whether 1 the regional reservoir age of surface waters was invariably 641 119 years through time and or 2 sbb laminae couplets are consistently annual varves and counted with sufficient precision this high resolution 14 c study of sbb sediments compares 49 mixed planktonic foraminiferal carbonate and 20 terrestrial organic carbon 14 c dates to the varve chronology to extend the high resolution paleoclimate chronology of the basin back 2000 years evidence indicates that regional reservoir ages do not remain constant through time with r i e regional reservoir age minus variations in the global mixed layer ocean reservoir age fluctuating between 80 and 350 years second there is a consistent r 2 0 96 undercounting of the varves between 150 and 1700 ad based on a new varying r 14 c chronology indicating that some laminae couplets may not be annual loss of varves may occur when low riverine input and infrequent winter storm activity during drought intervals fail to supply a siliciclastic layer

radiolarian assemblages from santa barbara basin sediments recent interdecadal variability santa barbara basin contains a sedimentary record ideal for high resolution paleoclimate studies because of the annual varves and regional to global scale climate signals preserved in the sediments lange et al 1990 kennett and ingram 1995 even though it does not lie directly in the path of the california current a nearly 100 year annual time series 1909 1991 of polycystine radiolarian assemblages from santa barbara basin sbb sediments was analyzed to the species level counts on a replicate sbb core dated 1870 1987 indicate that results are reproducible and the flux of a few representative species can be extrapolated to estimate fluxes of environmentally sensitive groups the frequency of species occurrences resembles a lognormal curve and year by year comparisons of species fluxes revealed only modest changes in the assemblages from 1909 1991 indicating that the assemblages represent a single oceanic province for paleoceanographic analysis of the radiolarian record species were combined into groups according to the water mass in which they occur to test this method temperature sensitive species were identified using t tests this generated warm and cool classes exhibiting trends in relative flux similar to those of the water mass groups both total flux and relative fluxes of water mass groups relate to low frequency decadal scale temperature fluctuations but not strongly to el nino southern oscillation events generally fluxes of species from different water masses covary suggesting changing carrying capacities and productivity through time while the consistent inverse relationship in relative fluxes indicate variability in climate the subtle decadal scale changes in assemblages diversity and increase in percent warm water fauna are consistent with a spin down of the california current system suggested by other records

hyperpycnal sediment discharge from semiarid southern california rivers implications for coastal sediment budgets southern california rivers discharge hyperpycnal river density greater than ocean density concentrations of suspended sediment 40 g l according to buoyancy theory during flood events mostly during el nino southern oscillation enso conditions because hyperpycnal river discharge commonly occurs during brief periods hours to occasionally days mean daily flow statistics often do not reveal the magnitude of these events hyperpycnal events are particularly important in rivers draining the transverse range and account for 75 of the cumulative sediment load discharged by the santa clara river over the past 50 yr these events are highly pulsed totaling only 30 days 0 15 of the total 50 yr period observations of the fate of sediment discharge although rare are consistent with hyperpycnal river dynamics and the high likelihood of turbidity currents during these events we suggest that much of the sediment load initially bypasses the littoral circulation cells and is directly deposited on the adjacent continental shelf thus potentially representing a loss of immediate beach sand supply during particularly exceptional events 100 yr recurrence intervals flood underflows may extend past the shelf and escape to offshore basins

inter decadal signals during the last millennium ad 1117 1992 in the varve record of santa barbara basin california annual varve thickness ad 1117 1992 from santa barbara basin has been decomposed into orthogonal components using singular spectrum analysis ssa to identify and retrieve inter decadal oscillations after removing all variability with periods greater than 150 years leading ssa eigenfunctions eofs identify four oscillatory pairs with periods of 100 58 25 and 12 years respectively based on 2500 simulated series and on two sided confidence intervals eofs 1 7 are significant at the 99 level and eofs 8 9 are significant at the 90 level oscillatory signals retrieved from the marine varves show an abrupt change in frequency and amplitude near ad 1600 the largest contribution to this environmental shift is given by the interdecadal components especially the 25 and the 12 year oscillation the near ad 1600 change may be related to multi annual events reported in the stratigraphy of the nearby santa monica basin and in dendrochronological records of the american southwest

holocene foraminiferal radiocarbon record of paleocirculation in the santa barbara basin differences in radiocarbon apparent ages between coexisting planktonic and benthic b p foraminifers in the santa barbara basin contain information about changes in the age and thus the source of water entering the basin the holocene sequence indicates a greater variability in circulation than previously detected during the early holocene ca 9 ka b p 1 4 c apparent age differences fluctuated between 220 and 560 1 4 c yr similar to rapid variations seen during the younger dryas and the last glacial episode during the relatively stable middle holocene 8 5 2 ka b p 1 4 c apparent age differences averaged 440 1 4 c yr after 2 ka the average b p 1 4 c apparent age difference increased to 620 1 4 c yr which has persisted until the present the shift in b p 1 4 c apparent age differences likely represents a change in the strength or source of north pacific intermediate water production and is concurrent with changes in regional climate patterns as well as with changes in temperature and accumulation rates measured in the greenland ice sheet project 2 ice core suggesting large scale changes in atmospheric circulation patterns

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity