The Neoglaciation continues from the Mid Holocene in to the Late Holocene. A continued decreasing trend of δO18 is seen throughout this time period, indicating warming to present day temperatures.
Within the foraminiferal assemblages, there is a large and sudden peak of C. wuellerstorfi, and decease of I. norcrossi abundances, from 3.4-3.9Ka BP. These changes occur at the same time that a decrease of ΔSS is observed in the sediment data. However, δO18 remains fairly constant at this time with only a small increase, and while δC13 is high, it is showing a decreasing trend from its previous
41
peak at 4.9Ka BP. This may indicate a “mini inter-glacial” period within the Neoglacial, indicating a retreat of the ice sheets. As the bottom current appears reduced at this time, perhaps there is a lag of organic carbon reaching the sea floor.
5.3.1 2000Yrs-Present
For the earliest Holocene to present it is possible to use data from both sediment cores, and compare spatially how the paleoclimate developed across the Nordic Sea. Five significant climatic events have been identified from the isotope data collected from C. wuellerstorfi sampled in core HH12-1206BC and are discussed below.
5.3.1.1 1653Yrs BP
The first climatic event that is identified in the very Late Holocene occurs at 1653Yrs BP, and sees a large decrease of δC13, with a value of 0.82‰. This corresponds to peaks in % abundance of H.
elegans, C. teretis and I. norcrossi in core HH12-1206BC, while C. wuellerstorfi has a small peak during this time in core HH13-103MC. Globobulimina sp. disappears completely at this time from core HH13-103MC suggesting a higher oxygen content of the sediment from this time to present.
However this contrasts to the peaks of H. elegans and C. teretis found in core HH12-1206BC. The δC13 levels measured in core HH13-103MC are also seen to increase during this time, contradicting the values seen in HH12-1206BC. Therefore it is possible that the Svalbard shelf experienced a period of cooling, with lower oxygen levels and organic carbon content of the sediment, and increased ice cover due to the peaks of the above benthic species, while sediments further south in core HH13-103MC remained stable, with perhaps a small increase of NADW seen by increasing ΔSS and abundance of C. wuellerstorfi.
This cooling period corresponds to the Dark Ages Cold Period (DACP) from 400-800AD which would occur at ~1500-1100Yrs BP in the core. The DACP is due to a reduction of Atlantic Water (AW) and a stronger influence of Arctic Waters across the Arctic Front. (Mayewski et al., 2004; Spielhagen et al., 2011) The cold period found in the isotope record of this study is slightly earlier than found in other records which may be due to bioturbation or turbidity currents on the shelf slope reworking the sediment records.
5.3.1.2 632-608Yrs BP
The second climatic event identified occurs between 632-608Yrs BP, with a large decrease of both δC13 and δO18 across this time period in core HH12-1206BC. A peak of δO18 occurring just before at 632Yrs BP can be correlated across to δO18 values in HH13-103MC which also shows a peak at this
42
Figure 19: Temporal correlation of δO18 and δC14 records from C. wuellerstorfi collected in sediment cores HH13-103MC and HH12-1206BC. Red lines mark significant climatic events within the record.
time, suggesting a wide spread but short lived period of cooling, followed by a rapid period of warming. This could indicate the start of the Little Ice Age. Commencing at ~800Yrs BP, this was a period of cooling within the Holocene, with increased precipitation and advances of glaciers on Svalbard (Martin-Moreno, Alvarez and Hagen, 2017). From 872Yrs BP there is a general increasing trend seen in HH12-1206BC of δO18, reaching a peak at 632Yrs BP, indicating a gradual cooling trend towards the Little Ice Age. However, it does not last long, with levels of δO18 dropping very quickly again from 620Yrs BP.
43
An increased abundance of I. norcrossi from 680-632Yrs BP could indicate a greater presence of sea ice, of the advancing proximity of the ice sheet. However these values are not stable and fluctuate between 7-15% abundance, suggesting a period of unstable climate. From 872Yrs BP C. arctica also sees a general increasing trend of abundance, peaking at 572Yrs BP with 16.7%. This increase could follow the advance of the DACP, as this species prefers temperatures of -0.2oC.
Within core HH13-103BC, an increase of δO18 can be seen in the time period of ~8-600Yrs BP, meaning that the start of the Little Ice Age also effected temperatures further south on the Jan Mayen Ridge, perhaps with an colder inflow of the EGC becoming stronger at this time.
5.3.1.3 500Yrs BP
At 500Yrs BP, there is a peak of δO18 of 3.9‰, suggesting a short colder period. Coupled with
extremely low values of δC13 of 0.75‰. At this time low abundances of C. wuellerstorfi are apparent, with a peak of I. norcrossi, supporting a colder time period with a closer proximity to the sea ice edge. It is suggested that this data represents the end of the LIA, as the trend seen in the record after this time is generally that of warming. Dates of the LIA are varied within the literature, and so it is possible that the cooling trend continued further than previous thought in this location due to its high latitude positon. Reconstructions by Kinnard et al. (2011) supports this, and also suggests temporary short periods of warming within the LIA which would match the isotope records of this study. Decreasing δO18 at 548 and 668Yrs BP could indicate responses to the advection of AW into the arctic on a decade scale as well as a centennial scale.
5.3.1.4 366-339Yrs BP
At this time period, a sharp increase of δO18 is observed, preceding this is a low level of δO18, with climatic turn around occurring within a ~300 year time span. Changes are also observed within the foraminifera abundances, with lows of H. elegans, R. nodulosus and I. norcrossi. There is a dramatic change also of C. arctica, which within ~500 years, from 366-313YrsBP disappears completely from the sediment record. As an indicator species of colder water temperatures, the disappearance of C.
arctic would point to warmer sea bed conditions. However this is in direct contrast to the large increase of oxygen isotopes at this time. It is possible that there is a lag in the response of C. arctica, which is effected by the decreasing levels of δO18 from 399-366Yrs BP.
5.3.1.5 45Yrs BP
At 45Yrs BP there is a fourth large decline of δC13 found within the isotope record. This decline is not visible within core HH13-103MC due to the reduced sedimentation rate of HH13-103MC. Therefore
44
it is not possible to correlate this event with environmental conditions further south in core HH13-103MC. This event is seen in the foraminifera record as a disappearance of C. teretis, and low abundances of H. elegans, I. norcrossi and C. wuellerstorfi. R. nodulosus however shows a large peak of abundance, reaching 23.8%. R. nodulosus is an agglutinated species of foraminifera, and this important distinction between the species could explain the differences in percentage abundances observed in the assemblage record; carbonate dissolution. The dissolution in this area is most likely caused by the presence of cold saline bottom waters. A high level of organic carbon production during spring and summer, combined with brine production during the formation increases the concentration of CO2 in the surface waters, which is then transported downwards with the brines as they are denser than surrounding water masses. Sea ice cover would also restrict gas exchanges with the atmosphere, leading to the formation of a sink for atmospheric CO2 (Steinsund and Hald, 1993;
Murray and Alve, 1998).
Overall, core HH12-1206BC has a much higher rate of agglutinated foraminifera in comparison to HH13-103MC, which could be due to the different positions of the core locations. However it is difficult to make a true comparison due to the different age ranges of the two cores.
45 Chapter 6 - Conclusion
Correlation of two sediment cores of distinctly different ages and locations was attempted to be able to reconstruct paleoclimate and palaeoceanography of the Nordic Seas throughout the Holocene.
This was met with limited success, due to the large differences of sedimentation rate found between the sample cores.
Core HH13-103MC spans nearly the whole Holocene time period, from 0-11.5KaBP. Through the Early Holocene (11.5-8.2KaBP) temperatures are unstable. Fluctuating δO18 and δC13 from 11.5-10.6Ka shows an overall decrease of temperature, matched by the benthic foraminiferal record with low abundances of C. wuellerstorfi and high abundances of Globobulimina sp. and H. elegans. Large peaks of IRD flux are apparent, which are seen as tephra layers within the core and match known volcanic eruptions such as the Vedde Ash layer at 10.2KaBP. A strongly positive ΔSS correlates to cooling temperatures and decreased mixing of the water column inferring strong bottom currents.
The Mid Holocene is relatively stable, with low abundances of C. wuellerstorfi indicating the 8.2KaBP cooling period, before the Holocene Thermal Maximum can be seen reflected in the decrease of the cold loving species Globobulimina sp.. Towards the end of the Mid Holocene increasing abundancy of I. norcrossi implies the start of the Neoglacial period, with periods of advancing glaciers and/or sea ice.
Increased warming continues throughout the Late Holocene, with stable levels of IRD and Benthic Flux. Decreasing I. norcrossi suggests that the ice front is pushed further north.
From 2KaBP attempts have been made to correlate both sediment cores to achieve a spatial idea of changing palaeoceanography. Two separate dates can be correlated across the cores, ~1.6KaBP and
~600YrsBP, with both sharing low levels of δC13, suggesting poor levels of stratification in the water column across the Nordic Seas. Both of these dates can be attributed to the Dark Ages Cold Period.
Correlations after these dates becomes more difficult due to the different rates of sedimentation, 47Yrs/cm and 313Yrs/cm for HH12-1206BC and HH13-103MC respectively.
Therefore reconstruction of the Late Holocene is primarily made through interpretations of sediment core HH12-1206BC. From 45YrsBP to Present isotope levels can be seen reaching the Modern Maximum, suggesting that water masses have changed in the last ~100Yrs with increasing temperatures. Although it is not known for sure if this rise of temperature is a product of changing water masses of increased warm AW advection or due to anthropological influences.
46 Chapter 7 - Future Work
While work has been completed on both of the cores discussed in this study, there are multiple options of how work could continue to gain further insight. While both Oxygen and Carbon isotopes have been measured there is possibility for temperature reconstructions using transfer functions, would give a much deeper insight into the changing bottom water conditions, and if these had an impact on the benthic foraminifera assemblage.
There was also a great difficulty obtaining AMS dates for core HH13-103MC, which has severely decreased the accuracy of this study. With time, dates for this core could be collected and applied to give a more accurate correlation of the two cores, and allow a greater context to the other data collected for HH13-103MC.
Due to previous studies in the area and ongoing projects at the time of writing, tephra layers were not included in this study, except for dating purposes. This could be another point of interest for core HH13-103MC given its close proximity to both Jan Mayen and Iceland.
Further data to be collected would be the Total Organic Carbon (TOC). While it was not sampled for this study, samples collected from core HH13-103MC are still available for this purpose and would add to further studies. Samples are also available for the collection and study of planktic foraminifera.
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