intertropical latitudes and precessional and half precessional cycles a mcintyre and b molfino in their report about the forcing of atlantic equatorial and subpolar millennial cycles by precession suggest that climatic changes in high polar latitudes which are related to heinrich iceberg surges may be caused by events that occur in low latitudes 1 1 this

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

forcing of atlantic equatorial and subpolar millennial cycles by precession abundance cycles of the marine alga florisphaera profunda centered on a period of 7600 carbon 14 years 8400 calendar years are present in high resolution records from the equatorial atlantic spanning 0 to 45 000 years ago these cycles correlate with heinrich events 1 through 5 which document rapid changes in continental ice melting around the subpolar north atlantic these variations in f profunda are a direct response to modulation in zonal wind driven divergence produced by a precessional component of orbital variation during a time of reduced eccentricity modulation

stochastic forcing of pleistocene ice sheets implications for the origin of millennial scale climate oscillations natural climate variability with small 0 5w m 2 stochastic changes in insolation and examine the response in terms of ice volume change in an ensemble of coupled energy balance ice sheet model runs the total change in ice mass is comparable to a third of the volume of the current greenland ice sheet with meltwater discharge potentially sufficient to trigger dansgaard oeschger type events in general the eurasian ice sheet is more sensitive to perturbations than the north american ice sheet the distribution and occurrence of spectral peaks in the 1 9 kyr band is also comparable to the record of millennial oscillations over the last 500 000 years in a subpolar north atlantic core our results suggest that considerable caution is necessary in interpreting the physical significance of sub milankovitch spectral peaks in paleo time series index terms 4215 oceanography general climate and interannual variability 3309 4227 oceanography general diurnal seasonal and annual cycles 3309 meteorology and atmospheric dynamics climatology 1620 4267 oceanography general paleoceanography keywords dansgaard oeschger cycles stochastic ice sheets millennial variability citation hyde w t and t j crowley stochastic forcing of pleistocene ice sheets implications for the origin of millennial scale climate oscillations paleoceanography 17 4 1067 doi 10 1029 2001pa000669 2002

millennial scale fluctuations in subtropical northwestern atlantic surface ocean hydrography during the mid pleistocene 1 we use planktonic foraminiferal globigerinoides sacculifer and neogloboquadrina dutertrei 18o records from subtropical north atlantic site 1056 to reconstruct millennial scale changes in sea surface hydrography during marine isotope stages mis 10 through 12 340 440 ka the difference in the 18o records provides a measure of the thermal gradient from the mixed layer to the seasonal thermocline tml tc which we use to constrain upper ocean stratification and infer northward ocean heat transport late holocene g sacculifer and n dutertrei 18o values place controls on how the foraminifera record recent ocean conditions at site 1056 in the down core record we find extreme excursions toward g sacculifer 18o maxima and tml tc minima throughout the entire time interval these cold events have a similar amplitude throughout but are more frequent during the glacial than the interglacial intervals during interglacial mis 11 the excursions provide evidence for repeated cooling of the sea surface contributing to existing evidence for relative climate instability during interglacial intervals during mis 10 millennial scale variability in the subtropical record is very similar to that observed in the subpolar north atlantic at site 980 perhaps related to a common forcing such as a large northern hemisphere ice sheet within the limitation of the age models the cold events of mis 11 are almost synchronous with benthic foraminiferal 13c minima at subtropical north atlantic site 1063 which suggests a general surface to deep ocean link via thermohaline circulation during this interval of time

millennial scale fluctuations in north atlantic heat flux during the last 150 000 years abstract surface water and deep ocean palaeoclimate records obtained from north atlantic core su90 03 40 n 32 w exhibit pronounced fluctuations at sub orbital time scales surface temperature variability in the mid latitude atlantic over the last 150 kyr is dominated by forcing at precession 21 kyr and semi precession harmonics 11 kyr abrupt changes in sea surface temperatures show that the northward transport of heat by the north atlantic current was curtailed during ice rafting events sea surface temperatures were about 12 c colder than modern values during the most intense cooling episodes associated with major ice rafting events heinrich events 1 4 6 and 11 and 10 c below modern values during the later part of isotope stage 3 40 30 ka during these cold intervals the north atlantic current was displaced to south of 40 n as the cold meltwaters penetrated southwards into what is now the subtropical ocean these changes in surface ocean heat exchange were most probably allied to a latitudinal migration and increase in zonality of atmospheric circulation patterns concurrent shifts in ird concentration sea surface temperatures and benthic 13 c values provide evidence of coupling between sea surface processes and the deep water circulation and suggest that decreases in surface temperature and salinity during ice rafting events culminated in a significant reorganisation of north atlantic deep water production

research into the multifractal of earthquake spatial distribution abstract the spectra dq of the henon map are calculated and analyzed using the extended g p method the key problems such as the scaling range determination data sample selection and the calculation method are also discussed results show that the scaling radius r should be less than about 1 3 of the diameter of the research object e g the earthquake region for reducing the boundary effect and a sample volume of 500 may be sufficient for the small absolute of the q value we also calculate and study the whole multifractal spectra dq of available seismic data for example the catalogue of earthquakes which have the occurred in tangshan region north china the results show that there is a multifractal in the data and that the dq value showed a remarkable variation with time before the large tangshan earthquake of july 1976

abrupt wind regime changes in the north atlantic ocean during the past 30 000 60 000 years 1 the inputs of higher plants in blake outer ridge subtropical western north atlantic during marine isotope stage 3 mis3 have been recorded at high resolution by quantification of c23 c33 odd carbon numbered n alkanes and c20 c30 even carbon numbered n alkan 1 ols in sediment sections of ocean drilling program site 1060 the changes of these proxies at this open marine site are mainly related to eolian inputs their concentrations and fluxes exhibit major abrupt variations that are correlated with dansgaard oeschger d o patterns in greenland ice cores the ratios between interstadials and stadials range between 2 and 9 times the intense flux increases in the d o stadials are linked to strong enhancements of the westerly wind regime at these subtropical latitudes during stadials the observed variation was paralleled by changes in wind blown dust and the polar circulation index in greenland ice which is in agreement with previously hypothesized atmospheric teleconnections between northern and middle low latitudes of the northern hemisphere the close correspondence between sedimentary and ice core proxies is evidence that crossings of the glacial climate thresholds involved major reorganizations of the troposphere the observed large rise in higher plant biomarkers indicates that climate stabilization in the d o stadial conditions led to main increases in wind intensity

western north atlantic evidence for millennial scale changes in ocean circulation and climate two late quaternary series of high resolution percent carbonate data from western north atlantic sediment drifts bermuda rise and bahama outer ridge show millennial scale oscillations superimposed on the familiar longer period oscillations of orbital origin the dominant high frequency oscillation in these records has a quasi period of about 4000 years these percent caco3 changes most likely result from the influence of climate change on the flux of terrigenous material from eastern canada the resuspension of continental margin sediment by deep eddy kinetic energy and carbonate dissolution sediment is transported to the bermuda rise by deep recirculating gyres and to the bahama outer ridge by the deep western boundary current system stable isotope results on foraminifera across several of these oscillations from interstadial climate conditions and from a glacial inception display variability similar to that of percent caco3 oxygen isotope ratios of planktonic foraminifera suggest large variations in near surface temperature and or salinity and carbon isotope ratios of benthic foraminifera indicate that there were significant oscillations in the flux of north atlantic deep water nadw these data support models which couple surface ocean conditions in the north atlantic production of nadw north atlantic heat flux and evidence for temperature oscillations in ice cores

millennial scale changes in north atlantic deep water circulation during marine isotope stages 11 and 12 linkage to antarctic climate the time interval represented by marine isotope stages mis 11 and 12 ca 360 470 ka may contain the most extreme glacial and interglacial climate conditions of the late pleistocene sediments from the bermuda rise western north atlantic provide clues to the nature of climate variability during this period our geochemical records indicate that millennial scale climate instability and associated changes in north atlantic deep water nadw production occurred during both interglacial mis 11 and glacial mis 12 stage 12 is punctuated by a series of interstadial events that occurred at a 5 6 k y periodicity and the occurrence of ice rafted debris at various times during stage 12 indicates that icebergs were present at least as far south as 34 n during this glacial period within the limits of our correlation the atmospheric temperature changes recently reported for the vostok ice core for the stage 11 time period appear to be represented by coeval changes in nadw flow specifically warming in antarctica is associated with increased production of nadw

early onset and tropical forcing of 100 000 year pleistocene glacial cycles between 1 5 and 0 6 myr ago the period of the earth s glacial cycles changed from 41 kyr the period of the earth s obliquity cycles to 100 kyr the period of the earth s orbital eccentricity1 2 which has a much smaller effect on global insolation the timing of this transition and its causes pose one of the most perplexing problems in palaeoclimate research3 here we use complex demodulation to examine the phase evolution of precession and semiprecession cycles the latter of which are phase coupled to both precession and eccentricity in the tropical and extra tropical atlantic ocean we find that about 1 5 myr ago tropical semiprecession cycles with periods of about 11 5 kyr started to propagate to higher latitudes coincident with a growing amplitude envelope of the 100 kyr cycles evidence from numerical models suggests that cycles of about 10 kyr in length may be required to explain the high amplitude of the 100 kyr cycles4 combining our results with consideration of a modern analogue we conclude that increased heat flow across the equator or from the tropics to higher latitudes around 1 5 myr ago strengthened the semiprecession cycle in the northern hemisphere and triggered the transition to sustained 100 kyr glacial cycles

millennial scale climate change and oceanic processes in the late pliocene and early pleistocene we generated 200 500 year resolution records of oceanic processes in the north atlantic ocean drilling program site 983 60 24 n 23 38 w 1983 meters water depth for intervals in the latest pliocene 1 86 1 93 ma and the earliest pleistocene 1 75 1 83 ma in order to examine the linkages between millennial scale variations in the ocean and background glacial interglacial climate change within glacial intervals we find evidence for variations similar to those observed in the late pleistocene we find discrete ice rafted debris ird events that reoccur every 2 5 kyr these events are preceded by a short cooling and accompanied by a reorganization of glacial deep waters the timing of ird events in the late pliocene and early pleistocene intervals is similar to that of dansgaard oeschger cycles but we find no ird events comparable in timing to late pleistocene heinrich events although interglacial intervals are much more stable we do find evidence for low amplitude variations in deep water properties that reoccur every 2 kyr within interglacial intervals the similarity between our late pliocene early pleistocene records and late pleistocene records implies that the mechanism driving millennial scale variations cannot be uniquely attributed to the strongly nonlinear linkage between climate and insolation and the large ice sheets of the late pleistocene

persistent suborbital climate variability in marine isotope stage 5 and termination ii new surface water records from two high sedimentation rate sites located in the western subtropical north atlantic near the axis of the gulf stream provide clear evidence of suborbital climate variations through marine isotope stage mis 5 persisting even into the warm peak of the interglaciation substage 5e we found that the amplitude of suborbital climate oscillations did not vary significantly for the whole of mis 5 implying that ice volume has little or no influence on the amplitude of suborbital climate variability in this region although some records suggest that longer suborbital variations 4 10 kyr during mis 5 are linked to deepwater changes none of the existing records is of sufficient resolution to assess if a linkage occurred for oscillations shorter than 4 kyr however when examined in conjunction with published data from the norwegian sea new evidence from the subpolar north atlantic suggests that coupled surface deepwater oscillations occurred during the penultimate deglaciation this supports the hypothesis that during glacial and deglacial times ocean ice interactions and deepwater variability amplify suborbital climate change at higher latitudes we suggest that during the penultimate deglaciation the north atlantic deepwater source varied between nordic sea and open north atlantic locations in parallel with surface temperature oscillations

millennial scale variability in western subtropical north atlantic surface and deep water circulation during marine isotope stages 11 and 12 1 marine isotope stages miss 11 and 12 460 380 ka contain what may be the most extreme climate conditions of the last 500 000 years and occurred at a time when earth received a similar amount of solar insolation as the most recent glacial interglacial cycle two high deposition rate sediment cores ocean drilling program sites 1059 31 6 n 75 4 w 2985 m and 1055 32 6 n 76 2 w 1799 m from the western north atlantic were studied in order to examine the nature of millennial scale variability in north atlantic surface and thermohaline circulation during this time interval detailed records of 18o and 13c in the benthic foraminifera species cibicidoides wuellerstorfi and the planktonic species globigerinoides ruber white as well as detailed records of carbonate content were generated for both sites these records suggest rapid variability in the strength of north atlantic deep water production throughout miss 11 and 12 most of the observed changes in deep water circulation appear to be associated with or coupled to changes in sea surface hydrography the duration of interglacial conditions during mis 11 20 000 30 000 years is much longer than subsequent interglacial stages combined 18o measurements and mg ca based temperature estimates from g ruber suggest sea surface temperatures increased 2 3 c across the mis 12 mis 11 transition

sub milankovitch cycles in periplatform carbonates from the early pliocene great bahama bank high resolution bulk sediment magnetic susceptibility and aragonite content and 18o records from two different planktonic foraminifera species were analyzed in an early pliocene core interval from the straits of florida ocean drilling program site 1006 the 18o record of the shallow dwelling foraminifera g sacculifer and the aragonite content are dominated by sub milankovitch variability in contrast magnetic susceptibility and the 18o record of the deeper dwelling foraminifera g menardii show precession cycles the relationship between the aragonite and the paleoproxy data suggests that the export of sediment from the adjacent great bahama bank was triggered directly by atmospheric processes rather than by sea level change we propose a climate mechanism that bears similarities with the semiannual cycle component of eastern equatorial pacific sea surface temperatures under present day conditions

relation of subtropical atlantic temperature high latitude ice rafting deep water formation and european climate 130 000 60 000 years ago abstract a new high resolution record of sea surface temperature from the subtropical western north atlantic documents a series of abrupt coolings within marine isotope stage 5 which can be objectively correlated with marine core evidence for increased ice sheet discharge at subpolar latitudes and reduced north atlantic deep water formation these results indicate that ice sheet forcing of deep ocean circulation influenced surface temperatures over much of the north atlantic a proposed correlation to the pollen record of grande pile france indicates that each cold event seen in the pollen sequence has a unique counterpart in the record of subtropical ocean temperature if this correlation is correct it suggests that the warmest part of the european eemian ended suddenly in response to oceanographic changes and that the subsequent post temperate phase of the eemian extended well into the interval of ice sheet growth corresponding to marine isotope substage 5d

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

abrupt climate events 500 000 to 340 000 years ago evidence from subpolar north atlantic sediments subpolar north atlantic proxy records document millennial scale climate variations 500 000 to 340 000 years ago the cycles have an approximately constant pacing that is similar to that documented for the last glacial cycle these findings suggest that such climate variations are inherent to the late pleistocene regardless of glacial state sea surface temperature during the warm peak of marine isotope stage 11 mis 11 varied by 0 5 to 1 c less than the 4 to 4 5 c estimated during times of ice growth and the 3 c estimated for glacial maxima coherent deep ocean circulation changes were associated with glacial oscillations in sea surface temperature

gulf stream and western boundary undercurrent variations during mis 10 12 at site 1056 blake bahama outer ridge abstract the stable isotopic composition of two planktonic foraminifer species globigerinoides sacculifer and neogloboquadrina dutertrei and two benthic foraminifer species cibicidoides wuellerstorfi and uvigerina peregrina was measured at sub orbital resolution through the marine isotope stages miss 10 11 and 12 345 460 ka at site 1056 on the blake bahama outer ridge planktonic foraminifers were counted for the interval 405 450 ka at 2 4 kyr resolution site 1056 32 29 n 76 20 w is located on the continental slope water depth 2167 m beneath the gulf stream the average rate of sediment accumulation through the interval is 11 4 cm kyr but sediment accumulation is much more rapid during glacial intervals 15 17 cm kyr the decline in percent carbonate during glacial intervals and its rise during interglacials indicates that the increased sediment supply is of terrigenous origin low carbonate values and low benthic 13c which are both associated with a weak western boundary undercurrent and low north atlantic deep water production persist for 6 kyr after the benthic 18o record indicates that ice volume has begun to decrease recovery of carbonate and benthic 13c values is abrupt and rapid millennial scale variation 3 4 kyr is apparent in the glacial intervals of the planktonic 18o records and is more pronounced in a 18o record which represents the temperature range in the photic zone semi precessional 10 12 kyr cycles are apparent in the planktonic 13c record the millennial scale cycles are largely caused by an increase in g sacculifer 18o and represent surface warming they are interpreted as representing periodic increases in westward intensification of the gyre the semi precessional cycles are driven by changes in the n dutertrei 13c and represent fluctuations in the gulf stream itself and therefore likely have a tropical origin planktonic foraminifer census counts did not show an expected response to one of the largest glacial interglacial transitions of the late pleistocene the most obvious change was an increase in faunal diversity during mis 12 2 the interval of maximum 18o values this suggests that cool slope water and warm subtropical gyre water penetrated a more sluggish gulf stream with greater frequency at this time the millennial scale maxima in the 18o record are accompanied by decreases in diversity which is consistent with the interpretation of surface warming during these events

millennial scale climate instability during the early pleistocene epoch climate proxy records of the past 100 000 years show that the earth s climate has varied significantly and continuously on timescales as short as a few thousand years 1 7 similar variability has also recently been observed for the interval 340 500 thousand years ago8 these dramatic climate shifts expressed most strongly in the north atlantic region may be linked to and possibly amplified by alterations in the mode of ocean thermohaline circulation4 5 6 7 8 9 here we use sediment records of past iceberg discharge and deep water chemistry to show that such millennial scale oscillations in climate occurred over one million years ago this was a time of significantly different climate boundary conditions not only was the early pleistocene epoch generally warmer but global climate variations were governed largely by changes in earth s orbital obliguity our results suggest that such millennial scale climate instability may be a pervasive and long term characteristic of earth s climate rather than just a feature of the strong glacial interglacial cycles of the past 800 000 years

on sharp spectral lines in the climate record and the millennial peak the existence of a narrow spectral line in the internal oscillations of the climate system would be a physical discovery of the first order thus the appearance of such a spectral peak with a q 20 near 1500 years period in the physical properties of the greenland ice core is intriguing bandwidth less than two cycles in 110 000 years apparently similar phenomena exist in some deep sea cores it is shown however that the peak appears at precisely the period predicted as a simple alias of the seasonal cycle inadequately under the nyquist criterion sampled at integer multiples of the common year if this peak is therefore discounted climate variability appears as expected to be a continuum process in the millennial band more generally however aliasing of high frequencies must be a concern in any core analysis

climate stability during the pliocene warm period 1 we present a high resolution climate record from a sediment core spanning an 80 kyr interval of time during the mid pliocene epoch when warmer conditions and lower global ice volume prevailed worldwide oxygen and carbon isotope analyses were made on benthic and planktonic foraminifera from odp site 981 in the north atlantic the amplitude and approximate recurrence interval of suborbital variations in these records are comparable to those of holocene and marine isotope stage 11 mis 11 records from the north atlantic we conclude that the mid pliocene warm interval was a time of relative climatic stability these results suggest that warmer climatic conditions alone may not necessarily enhance variability in the climate system a finding that may facilitate predictions of 21st century climatic response to anthropogenic warming

amplitude variations of 1470 year climate oscillations during the last 100 000 years linked to fluctuations of continental ice mass we describe amplitude variations of a 1470 year y signal in the oxygen isotope record from the gisp2 ice core as a function of continental ice mass as recorded in sea level variations the 1470 y signal is closely tied to dansgaard oeschger interstadial events which perturbed the climate system on a global scale large amplitude 1470 y oscillations occur only if continental ice mass exceeds a threshold level equivalent to sea level at approximately 45 m and are tied to times of change in ice mass minima of the 1470 y signal in the greenland ice are associated with short time intervals of relatively stable phases of sea level and with reduced climate variability in the entire periodicity range between approximately 550 5000 y

evolution of millennial scale climate variability during the mid pleistocene 1 we use the oxygen isotopic composition of planktonic foraminifera globigerinoides ruber white from ocean drilling program site 1058 in the subtropical northwestern atlantic to construct a high resolution 800 year climate record spanning the mid pleistocene climate transition 410 ka to 1350 ka we investigate whether or not millennial scale instabilities in the proxy record are associated with the extent of continental glaciation g ruber 18o values display high frequency fluctuations throughout the record but the amplitude about mean glacial and interglacial 18o values increases at marine isotope stage mis 22 880 ka and is highest during mis 12 these observations support that millennial scale climate instabilities are associated with ice sheet size time series analysis illustrates that these variations have significant concentration of spectral power centered on periods of 10 12 ka and 5 ka the timing of these fluctuations agrees well or coincides with the periodicities of the second and fourth harmonics respectively of precessional forcing at the equator an insolation based origin of the millennial scale instabilities would be independent of ice volume and explains the presence of these fluctuations before the mid pleistocene climate transition as well as during interglacial intervals e g mis 37 and 17 because the amplitude of the millennial scale variations increases during the mid pleistocene transition feedback mechanisms associated with the growth of large 100 ka paced polar ice sheets may be important amplifiers of regional surface water hydrographic changes

subtropical north atlantic temperatures 60 000 to 30 000 years ago a reconstruction of sea surface temperature based on alkenone unsaturation ratios in sediments of the bermuda rise provides a detailed record of subtropical climate from 60 000 to 30 000 years ago northern sargasso sea temperatures changed repeatedly by 2 to 5 c covarying with high latitude temperatures that were previously inferred from greenland ice cores the largest temperature increases were comparable in magnitude to the full glacial holocene warming at the site abrupt cold reversals of 3 to 5 c lasting less than 250 years occurred during the onset of two such events greenland interstadials 8 and 12 suggesting that the largest most rapid warmings were especially unstable

the mid pleistocene climate transition onset of 100 ka cycle lags ice volume build up by 280 ka the mid pleistocene climate transition mpt is the complex climatic change which brought the late pleistocene ice ages we explore the mpt in the time and frequency domains by new methods of time series analysis high resolution oxygen isotope records reveal that the ice volume related increase in 18o mean amplitude 0 29 0 05 1 transition midpoint 922 12 ka duration 40 9 ka significantly preceded the abrupt increase in the amplitude of the 100 ka cycle at 641 9 ka this finding can be quantitatively simulated using a simple ice bedrock model in which due to the additional ice the calving threshold is exceeded the simulated calving events prior to 650 ka are separated by 77 ka whereas after 650 ka they occur pseudo periodically with a mean period of nearly 100 ka the cause for the delay of the 100 ka calving cycle was not a slow bedrock relaxation rather the coincidental combination of insolation existing ice mass and bedrock depression