Mystery Interval Research Articles

Early onset of summer monsoon during the Mystery Interval (17.5–14.5 ka BP) in the East Asian Summer Monsoon area: Evidence from leaf alkane δ2H

Exploring the seasonal atmospheric processes involved with hydroclimate changes of the East Asian Summer Monsoon (EASM) during the Mystery Interval (MI, 17.5–14.5 ka BP) will enhance understanding of the EASM climate system. Here we report on the δ2H values of leaf wax n-alkanes (δ2Halk) in a wetland core retrieved from southern China. These values are relatively negative during the MI, contrasting to positive excursion recorded by stalagmite δ18O in nearby caves during the same period. Considering the overlap of the primary synthesis time of leaf waxes in deciduous woody plants and periods of precipitation δ2H alteration in the EASM (late spring to early summer), we interpret the relatively negative signal of δ2Halk in the wetland core as the early onset of summer rainfall during the MI. The early onset of summer rainfall was likely due to the early northeastward shifting of the Western Pacific subtropical high due to the low land-sea thermogradient between the Qinghai-Tibet Plateau and the West Pacific Ocean.

Hydroclimate variation during the Mystery Interval in the East Asian Summer Monsoon area

The inconsistency of paleoclimate records between the mid-low and high latitudes during the Mystery Interval (MI, 17.5–14.5 ka BP) is not well resolved. To provide a mechanism analysis, this study conducts spatial comparisons of hydroclimate changes in the Asian Summer Monsoon (ASM) region during the MI by reconstructing the paleohydrology of the south of East Asian Summer Monsoon (EASM) region using the molecular distributions and carbon isotope ratio of n-alkanes, as well as elemental and isotopic composition of bulk organic matter, in a sedimentary core retrieved from Dingnan (DN) wetland. The multiple indices of the DN core confirmed the occurrence of dry-wet and wet-dry transitions at ca. 17.5 ka BP and 16.0 ka BP, respectively. Synchronous with the dry-wet transition in the DN records, the Indochina Peninsula at a slightly lower latitude became wetter. In contrast, the middle Yangtze region at a more northern latitude became slightly drier, and North China did not show a hydrological variation. Moreover, the wet-dry transition at the mid-MI was strong in the Indochina Peninsula (10–20°N, at about 16.5 ka BP) and the southern EASM region (23–27°N, at about 16.0 ka BP) but was nearly absent in the middle Yangtze region (28–32°N) and North China (35–40°N). The first hydrological transition possibly connects with the synchronous southward movement of the Intertropical Convergence Zone (ITCZ) and the westerly jet caused by the cooling of the Northern Hemisphere. The absence of the second hydrological transition is interpreted as a result of the decoupling of the ITCZ and the westerly jet caused by the delayed formation of the ITCZ due to the warming of the Southern Hemisphere. This study reveals spatial differences in the hydroclimate variations during the MI and highlights the potential influence of coupling between the westerly jet and the ITCZ on rainfall in the ASM region.

Anti-phase Variation of Hydrology and In-Phase Carbon Accumulations in Two Wetlands in Southern and Northern China Since the Last Deglaciation

To examine the spatial patterns of hydrological variations in the southern and northern East Asia Monsoonal (EAM) region on millennial scales as well as to investigate the relations of hydrological changes on carbon accumulation in the regions with contrasting environmental backgrounds, we performed facies-based hydrological reconstructions at two wetlands, Midiwan wetland (37°39’N, 108°37’E) and Dahu wettland (24°45’N, 115°2’E), respectively, located in a semi-arid loess-desert transitional zone and humid southern China. Our reconstructions show that there is an anti-phase pattern of the precipitation in these two wetlands on a millennial scale. However, with the different responses to the contrasting hydrological conditions, the carbon accumulations at these two sites show in-phase patterns on a millennial scale. These results indicate that the carbon accumulations in the two sites are mainly controlled by the local hydrologic conditions. The wetlands in both southern and northern China were found to be expanding during the interval from 6 to 4 cal. ka BP (ka = kilo annum), as inferred by the higher total organic carbon content. For the Mystery Interval (MI, from 17.5 to 14.5 cal. ka BP), however, both hydrological conditions and carbon accumulations in these two sites showed an in-phase pattern.

Extrapolar climate reversal during the last deglaciation

Large ocean-atmosphere and hydroclimate changes occurred during the last deglaciation, although the interplay between these changes remains ambiguous. Here, we present a speleothem-based high resolution record of Northern Hemisphere atmospheric temperature driven polar jet variability, which matches the Greenland ice core records for the most of the last glacial period, except during the last deglaciation. Our data, combined with data from across the globe, show a dramatic climate reversal during the last deglaciation, which we refer to as the Extrapolar Climate Reversal (ECR). This is the most prominent feature in most tropical and subtropical hydroclimate proxies. The initiation of the ECR coincides with the rapid rise in CO2, in part attributed to upwelling in the Southern Ocean and the near collapse of the Atlantic Meridional Overturning Circulation. We attribute the ECR to upwelling of cold deep waters from the Southern Ocean. This is supported by a variety of proxies showing the incursion of deep Southern Ocean waters into the tropics and subtropics. Regional climate variability across the extropolar regions during the interval previously referred to as the “Mystery Interval” can now be explained in the context of the ECR event.

Surface vitrification caused by natural fires in Late Pleistocene wetlands of the Atacama Desert

We describe extended occurrences of unusual silicate glass surface layers from the Atacama Desert (Chile). These glasses, found near the town of Pica at four localities separated by up to 70 km, are neither fulgurites, nor volcanic glasses, nor metallurgical slags related to anthropic activity, but show close similarities to other glasses that have been previously attributed to large airbursts created by meteoroids entering the Earth's atmosphere. The glasses are restricted to specific Late Pleistocene terrains: paleo-wetlands and soils rich in organic matter with SiO2-rich plant remains, salts and carbonates. 14C dating and paleomagnetic data indicate that the glasses were formed during at least two distinct periods. This rules out the hypothesis of a single large airburst as the cause of surface melting. Instead, burning of organic-rich soils in dried-out grassy wetlands during climate oscillations between wet and dry periods can account for the formation of the Pica glasses. Large oases did indeed form in the hyperarid Atacama Desert due to elevated groundwater tables and increased surface discharge during the Central Andean Pluvial Event (roughly coeval with the Mystery interval and Younger Dryas). Finally, we discuss the implications of our results for the other surface glasses previously attributed to extraterrestrial events.

Lacustrine record of centennial- and millennial-scale rainfall variability of the East Asian summer monsoon during the last deglaciation: Multi-proxy evidence from Taiwan

Situated at the border between the Southeast Asian continent and the northwestern Pacific, Taiwan's climate is primarily influenced by the East Asian Summer Monsoon (EASM) and episodic tropical cyclones formed in the western Pacific. The lake sediments in Taiwan thus potentially archive the information of typhoon events and past monsoon variability. A high-resolution multi-proxy (total carbon, mass accumulation rate of total carbon, weight percent of wood fragments, δ13C of the organic carbon, magnetic susceptibility, chemical weathering index) and well-dated lacustrine record from Dongyuan Lake (22°10′N, 120°50′E; 360m above sea level) in southern Taiwan was used to reconstruct centennial to millennial timescale EASM oscillations specifically for the transition from the last deglaciation to the Early Holocene (17–9ka BP). The temporal patterns of proxies on both timescales in our records broadly agree with those in the Greenland ice core, which shows enhanced EASM during the warm period and weakened monsoon during the cold period. Different from northern high-latitude climates, we found two phases in the Mystery Interval, opposite Bølling–Allerød (BA) trend with maximum monsoonal rainfall in the Allerød period and more gradual transition to Younger Dryas event in our record. Likely, other climate forcings beyond the North Atlantic climate jointly modulated the EASM. Changes of sea surface temperature in the western tropical Pacific also might have exerted synergistic control on EASM precipitation in Taiwan on the centennial timescale during the last deglaciation. In addition, several high sedimentation events accompanied by significant amounts of wood fragments were observed and coincide with the paleo-record of mass wasting events identified on various downstream floodplains in Taiwan. Since mass wasting processes were rainfall-driven and threshold-triggered, such coherence possibly indicated an intensification of typhoon activity during the early Holocene. More high-resolution lacustrine records are required to decipher the hydrological variation and climate dynamics that were co-influenced by the Intertropical Convergence Zone (ITCZ) and typhoon activity in Southeast Asia.

Carbonate preservation during the ‘mystery interval’ in the northern Indian Ocean

Carbonate preservation during the ‘mystery interval’ in the northern Indian Ocean

Deep water exchanges between the South China Sea and the Pacific since the last glacial period

AbstractDeep ocean circulation is widely considered as one of the important factors for increasing CO2 concentration and decreasing radiocarbon activity (Δ14C) of the atmosphere during the last deglaciation. The AMS 14C ages of benthic and planktonic foraminifers from 18 samples of Core MD05‐2904 (water depth of 2066 m) in the northern South China Sea (SCS) and 15 samples of Core MD05‐2896 (water depth of 1657 m) in the southern SCS were analyzed in this study for reconstructing the intrabasin deep oceanic processes and hence exploring the deep water exchanges between the SCS and the Pacific since the last glacial period. The results show that during the Holocene the average apparent ventilation age of deep water was younger in the southern SCS (~1350 years) than in the northern SCS (~1850 years) due to relatively strong vertical mixing and advection, consistent with modern observations. However, during the last glacial period and deglaciation the deep water was older in the southern SCS (~2050 years and ~1800 to 1200 years, respectively) than in the northern SCS (~1600 years and ~670 years, respectively), indicating reduced deep mixing and advection. Moreover, the northern SCS deep water was significantly younger during the last deglaciation than during the Holocene and the last glacial period, implying the existence of northern sourced newly formed and relatively young North Pacific deep water. Our records do not support the intrusion of anomalously 14C‐depleted deep water to the middepth of the low‐latitude western Pacific and the SCS during the “Mystery Interval” (17.5–14.5 kyr B.P.).

A detailed East Asian monsoon history surrounding the ‘Mystery Interval’ derived from three Chinese speleothem records

The ‘Mystery Interval’ (MI, 17.5−14.5ka) was the first stage of the last deglaciation, a key interval for understanding mechanisms of glacial–interglacial cycles. To elucidate possible causes of the MI, here we present three high-resolution, precisely dated oxygen-isotope records of stalagmites from Qingtian and Hulu Caves in China, reflecting changes in the East Asian summer monsoon (EASM) then. Based on well-established chronologies using precise 230Th dates and annual-band counting results, the two-cave δ18O profiles of ~7-yr resolution match well at decadal timescales. Both of the two-cave records document an abrupt weakening (2‰ of δ18O rise within 20yr) in the EASM at ~16.1ka, coinciding with the transition of the two-phased MI reconstructed from New Mexico's Lake Estancia. Our results indicate that the maximum southward displacement of the Intertropical Convergence Zone and associated southward shift of polar jet stream may generate this two-phase feature of the MI during that time. We also discover a linear relationship among decreasing EASM intensity, rising atmospheric CO2 and weakening Atlantic Meridional Overturning Circulation between the MI and Younger Dryas episodes, suggesting a strong coupling of atmospheric/oceanic circulations in response to the millennial-scale forcing, which in turn regulates global climate changes and carbon cycles.

Peak glacial <sup>14</sup>C ventilation ages suggest major draw-down of carbon into the abyssal ocean

Abstract. Ice core records demonstrate a glacial–interglacial atmospheric CO2 increase of ~ 100 ppm, while 14C calibration efforts document a strong decrease in atmospheric 14C concentration during this period. A calculated transfer of ~ 530 Gt of 14C-depleted carbon is required to produce the deglacial coeval rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide Δ14C data set showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the maximum 14C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14C yr, in the extreme reaching 5100 14C yr. Below 2000 m depth the 14C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We propose as a working hypothesis that the modern regression of DIC vs. Δ14C also applies for LGM times, which implies that a mean LGM aging of ~ 600 14C yr corresponded to a global rise of ~ 85–115 μmol DIC kg−1 in the deep ocean. Thus, the prolonged residence time of ocean deep waters may indeed have made it possible to absorb an additional ~ 730–980 Gt DIC, one third of which possibly originated from intermediate waters. We also infer that LGM deep-water O2 dropped to suboxic values of < 10 μmol kg−1 in the Atlantic sector of the Southern Ocean, possibly also in the subpolar North Pacific. The deglacial transfer of the extra-aged, deep-ocean carbon to the atmosphere via the dynamic ocean–atmosphere carbon exchange would be sufficient to account for two trends observed, (1) for the increase in atmospheric CO2 and (2) for the 190‰ drop in atmospheric Δ14C during the so-called HS-1 "Mystery Interval", when atmospheric 14C production rates were largely constant.

Late Glacial and Holocene record of climatic change in the southern Rocky Mountains from sediments in San Luis Lake, Colorado, USA

Large rapid climate changes occurred over the last glacial cycle in the southwestern United States and elsewhere in many regions of the world. Some of these changes were attributed to alternations between stadial and interstadial conditions in the North Atlantic. But intense debate exists on how climate anomalies in the North Atlantic transmit to the southwest. Here we report a sediment record from San Luis Lake in southern Colorado, through analyses of grain size, magnetic susceptibility, Mg/Ca, total inorganic carbon, δ18O and δ13C, to indicate climatic and environmental changes in the southern Rocky Mountains over the last 16.5ka. We found that San Luis Lake remained hydrologically closed most of the time but overflowed during the second half of the Mystery Interval (the Big Wet: 15.7–14.9ka) and the latter part of the mid-Holocene (the Neopluvial: 4–3ka). Over the course of the last deglaciation, San Luis Lake underwent a series of large millennial-scale hydroclimatic changes such as the Big Dry (16.5–15.7ka), the Big Wet, the Bølling–Allerød dry (14.9–12.7ka), and the Younger Dryas wet (12.8–11.6ka), corresponding to warm/cold phases in the high-latitude Northern Hemisphere. The North American monsoon waxed during the Pre-Boreal interval (11.6–10.5ka) and waned through the Holocene, in phase with northward and southward displacement of the intertropical convergence zone (ITCZ). The San Luis Lake basin was relatively dry in the early Holocene (10.5–6.7ka), wet and fluctuating in the mid-Holocene (6.7–2.6ka), and dry and less variable in the late Holocene (2.6–0ka). We found evidence that extreme pluvial episodes of the southern Rocky Mountains and elsewhere in the American Southwest were coeval with cold phases of the North Pacific. Our results highlight the role of the North Pacific in modulating atmospheric circulations over the region on millennial timescales.

Diatom and vegetation responses to Late Glacial and Early Holocene climate changes at Lake Estanya (Southern Pyrenees, NE Spain)

We investigate Lake Estanya's diatom and pollen records from the Late Glacial (LG) to the Early Holocene (EH), in order to compare limnological and vegetation responses to common climate forcing. The biotic changes recognized in this study largely agree with the hydrological evolution of the lake described previously for the same period. The diatom record shows high sensitivity to fluctuations in both lake level and salinity concentration as a consequence of climate shifts. In addition vegetation results indicate that the area could have played an important role as regional vegetation refuge. Shallow lake conditions during the Last Glacial Maximum (LGM) were punctuated by relatively deeper freshwaters between 19.3 and 18.6calkyr BP and at 18.0calkyr BP, as recorded by diatom shifts. A subsequent increasing aridity trend, coinciding with the Mystery Interval (MI), affected the diatom accumulation rates, which dropped to its minimum values between 17.2 and 14.7calkyr BP. Particularly dry and cold conditions during the LGM and MI are supported by the largest values of steppic pollen taxa of the whole sequence, which account for up to 40%. However, relatively high values of Betula during the Heinrich Event 1 suggest a plausible regional vegetation refuge. Abrupt cooling and warming episodes within the LG triggered remarkable ecological threshold crossings in the diatom communities, especially during the stadial/interstadial episodes. At this point, the vegetation reflects the onset of warm conditions during the Bølling/Allerød with the partial substitution of Betula by Marcescent and Evergreen Quercus, which probably indicates the arrival of temperate taxa to the area and the likely migration of birch to higher altitudes. The Younger Dryas Stadial shows a complex ecological response. Diatoms are very poorly preserved, but aquatic taxa reach their highest values. An increase in Marcescent Quercus during this cold stage lends further support to the hypothesis that this is a regional vegetation refuge. Low lake levels recorded during the EH affected the development and preservation of diatom communities. A delay in the onset of humid conditions for the EH is also supported by the vegetation composition, characterized by the maximum expansion of Juniperus.

Oceanic carbon and water masses during the Mystery Interval: A model‐data comparison study

The ‘Mystery Interval’ (17.5–14.5 ka BP) is characterized by a large decline in atmospheric Δ14C synchronous with an increase in atmospheric CO2. The most widely accepted hypothesis to explain these observed shifts involves the existence of an isolated ‘old’ ocean carbon reservoir that was subsequently ventilated. Here we use the UVic Earth System Climate Model to locate a potential carbon rich and Δ14C depleted water mass under 17.5 ka BP boundary conditions. We then investigate two mechanisms for the potential ventilation of such a reservoir, namely the weakening of the North Atlantic Meridional Overturning due to iceberg calving and latitudinal shifts in Southern Hemisphere Westerlies (SHW) due to southern hemispheric warming. We find that simulations derived from an equilibrium state forced with present‐day SHW and moderate North Atlantic Deep Water (NADW) formation are in better agreement with atmospheric and ocean Δ14C reconstructions than simulations derived from an equilibrium state forced with a northward shifted SHW belt resulting in a shut‐down of the Atlantic Meridional Overturning and formation of North Pacific Deep Water. For simulations with present‐day SHW, the oldest water masses are found in the North Pacific, although the Southern Ocean cannot be ruled out as a potential ‘Mystery Reservoir’. According to our simulations, the strength of Atlantic overturning is the dominant mechanism in increasing the ocean‐atmosphere carbon flux, while shifting SHW results in a rearrangement of deep ocean carbon largely between the Atlantic and Pacific basins. In our ‘best case’ scenario, the model can account for 58% of the atmospheric CO2 increase and 48% of the atmospheric Δ14C decline. While the rate of ventilation and the age of ventilated water masses are comparable with observations, the ventilation in the model could not be sustained long enough to account for the full excursion seen in paleodata.

How did the hydrologic cycle respond to the two-phase mystery interval?

Lake Estancia's transition from a Big Dry episode during the first half of the Mystery Interval to a Big Wet episode during the second half has equivalents in records from across the planet. At the time of this transition, Chinese monsoons experienced pronounced weakening, closed-basin lakes in both the Great Basin of the western United States and in the southern Altiplano of South America underwent a major expansion, mountain glaciers in Southern Hemisphere middle latitudes had retreated, and the rates of increase of CO2 and of δ18O in Antarctic ice underwent a decrease. Finally, the precipitous drop in dust rain over Antarctica and the Southern Ocean terminated as did a similar drop in the 13C to 12C ratio in atmospheric CO2. These changes are consistent with a southward shift of the thermal equator. The cause of such a shift is thought to be an expansion of sea ice caused by a shutdown in deep water production in the northern Atlantic. This creates a dilemma because a similar southward shift is an expected consequence of the Heinrich event #1 which initiated the Mystery Interval.

Seasonal Laurentide Ice Sheet melting during the "Mystery Interval" (17.5-14.5 ka)

The last deglaciation in the Northern Hemisphere was interrupted by two major stadials, the so-called “Mystery Interval” (17.5–14.5 ka) and the Younger Dryas (12.9–11.7 ka). During these events, the North Atlantic region was marked by cold surface conditions, yet simultaneous glacier and snowline retreat. Rerouting of Laurentide Ice Sheet meltwater from the Gulf of Mexico to an eastern or northern spillway may have reduced meridional overturning circulation at the onset of the Younger Dryas. However, this hypothesis has not been tested for the Mystery Interval. Paired Mg/Ca and δ 18 O measurements on foraminifera from laminated Orca Basin sediments in the Gulf of Mexico, constrained by 35 14 C dates, document the timing of meltwater input with subcentennial resolution. Isolating the δ 18 O of seawater (termed δ 18 O GOM ) reveals three major melting episodes from ca. 17.5 ka until 12.9 ka, followed by a rapid cessation, consistent with meltwater rerouting at the onset of the Younger Dryas. Conversely, inferred meltwater fl ow to the Gulf of Mexico during the Mystery Interval does not support a simple routing event, but is consistent with glacier and snowline retreat. We suggest that summer melting of Northern Hemisphere ice sheets during this stadial may have been an important mechanism for enhanced winter sea-ice formation, hypercold winter conditions, and enhanced seasonality in the North Atlantic region.

A speleothem record of glacial (25–11.6 kyr BP) rapid climatic changes from northern Iberian Peninsula

Low and high frequency climatic fluctuations in northern Iberian Peninsula during the last glacial maximum (LGM) and deglaciation are documented in a stalagmite using δ 18O and δ 13C and hydrologically sensitive trace metal ratios Mg/Ca and Ba/Ca. U/Th dating indicates speleothem growth commenced at 25 kyr BP (Present = year 1950) and extended to 11.6 kyr BP making this one of few European speleothem growing during the last glacial period. Rapid climatic fluctuations as Heinrich event 2 (H2) and Greenland Interstadial (GI-) 2 are well characterized in this record by more arid and cold conditions and by more humid conditions, respectively. Speleothem growth ceased from 18.2 to 15.4 kyr BP (the so-called Mystery Interval) likely reflecting the driest and potentially coldest conditions of this record, coincident with the 2 kyr duration shutdown of the North Atlantic Meridional Overturning Circulation (MOC). A major gradual increase in humidity and possibly in temperature occurred from 15.5 to 13.5 kyr BP, beginning in the Bølling and culminating in the Allerød period. This gradual humidity change contrasts with more abrupt humidity shifts in the Mediterranean, suggesting a different climate threshold for Mediterranean vs. Atlantic margin precipitation.

A Great Basin-wide dry episode during the first half of the Mystery Interval?

The existence of the Big Dry event from 14.9 to 13.8 14C kyrs in the Lake Estancia New Mexico record suggests that the deglacial Mystery Interval (14.5–12.4 14C kyrs) has two distinct hydrologic parts in the western USA. During the first, Great Basin Lake Estancia shrank in size and during the second, Great Basin Lake Lahontan reached its largest size. It is tempting to postulate that the transition between these two parts of the Mystery Interval were triggered by the IRD event recorded off Portugal at about 13.8 14C kyrs which post dates Heinrich event #1 by about 1.5 kyrs. This twofold division is consistent with the record from Hulu Cave, China, in which the initiation of the weak monsoon event occurs in the middle of the Mystery Interval at 16.1 kyrs (i.e., about 13.8 14C kyrs).

Near constancy of the Pacific Ocean surface to mid-depth radiocarbon-age difference over the last 20 kyr

Although 13C to 12C and cadmium to calcium ratios provide information regarding the distribution of deep water masses during late glacial time and during the period of deglaciation, our knowledge of the rate at which these water masses were ventilated comes mainly from the difference in radiocarbon-age between coexisting bottom- and surface-dwelling foraminifera. Paired benthic/planktonic foraminiferal radiocarbon-age differences covering last 20 kyr in a high-deposition-rate western equatorial Pacific core, MD01-2386, from a water depth of 2.8 km show no significant climate-related variations over this period. This result is surprising for we would have expected a change in this age difference between the last glacial maximum (LGM) and the Holocene and also during the Mystery Interval (17.5–14.5 kyr ago) when the waters in a radiocarbon-depleted abyssal reservoir were presumably being mixed back into the remainder of the ocean.

Radiocarbon age of late glacial deep water from the equatorial Pacific

Radiocarbon age differences for pairs of coexisting late glacial age benthic and planktic foraminifera shells handpicked from 10 sediment samples from a core from a depth of 2.8 km in the western equatorial Pacific are not significantly different from that of 1600 years calculated from measurements on prenuclear seawater. This places a lower limit on the depth of the interface for the hypothetical radiocarbon‐depleted glacial age seawater reservoir required to explain the 190‰ drop in the 14C/C for atmospheric CO2, which occurred during the mystery interval (17.5 to 14.5 calendar years ago). These measurements restrict the volume of this reservoir to be no more than 35% that of the ocean. Further, 14C measurements on a single Last Glacial Maximum age sample from a central equatorial Pacific core from a depth of 4.4 km water fail to reveal evidence for the required 5‐ to 7‐kyr age difference between benthic and planktic foraminifera shells if the isolated reservoir occupied only one third of the ocean. Nor does the 13C record for benthic forams from this abyssal core yield any evidence for the excess respiration CO2 expected to be produced during thousands of years of isolation. Nor, as indicated by the presence of benthic foraminifera, was the dissolved oxygen used up in this abyssal water.

A 190‰ drop in atmosphere's Δ 14C during the “Mystery Interval” (17.5 to 14.5 kyr)

Reconstructions of atmospheric Δ 14C for the deglacial period reveal a 190 ± 10‰ drop between 17.5 and 14.5 kyr before present, during the ‘mystery interval’. While it appears that the major contributor must be the mixing between an isolated low radiocarbon abyssal reservoir and the remainder of the ocean, a combination of 14C measurements on coexisting benthic and planktic foraminifera shells and 13C/ 12C measurements on benthic foraminifera make it difficult to identify a sufficiently large volume for this reservoir. An alternative scenario, based on higher 14C production rates between about 38 and 18 kyr ago, would escape the need for a large, isolated reservoir prior to the mystery interval but instead would call for a better ventilated ocean afterwards. The addition of 14C-deficient sedimentary carbon appears not to be an important contributor. While the unlikely explanation that the half-life of 14C is 15% greater than the accepted value (5730 yr) would eliminate the steady decrease in the 14C/C over the last 40 kyr, it would not explain the steep drop which occurred between 17.5 and 14.5 kyr.