Variability in δ18O diatom during the past 6,250 years

Core PS67/182-1 recovered an 18-m sequence of laminated diatom-rich mud, deposited in a seasonally open marine environment (see Materials and Methods for more details). The sediment sequence was dated using both 210Pb and 14C methods, with age-depth modelling performed in CLAM v2.233 (see Materials and Methods, Fig. S6). The base of PS67/182-1 is dated to 6,250 calibrated years before present (where ‘present’ corresponds to A.D. 1950). Change point analysis was undertaken to identify the timing of significant changes in δ18O diatom data (see Materials and Methods, Fig. S7). The δ18O diatom record shows relatively uniform values from 6,250 to 1,617 cal. yrs BP (Fig. 2a). The first significant change points in the data show an increase in glacial discharge (i.e., trend to lower δ18O diatom values) between 1,617 and 720 cal. yr BP followed by a decline (Fig. 2a). Since c. 550 cal. yr BP (~A.D. 1,400) discharge has increased steadily with a significant change-point at 244 cal. yr BP (~A.D. 1706), where the magnitude of discharge emerges beyond the range of natural variability set during the past 6,250 years (Fig. 2a). A further significant change point at 38 cal. yr BP (A.D. 1912) (Fig. 2a) shows a marked acceleration in the rate of ice melt in the last 106 years. The apparent lack of correspondence between δ18O diatom and ice-rafted debris (IRD) counts (>2 mm) and/or sand fraction (Fig. S4g) particularly throughout this period of elevated glacial discharge suggests that the δ18O diatom signal at core site PS67/182-1 is mainly controlled by ice shelf, glacier and iceberg melting along the eastern AP rather than melting of icebergs directly over the core site. Other measured proxies also vary throughout the Holocene (Fig. S4) but there is no clear long-term relationship between the δ18O diatom record and these proxies. Bulk productivity indicators (TOC, bOpal, Ba/Ti) show centennial to millennial-scale variability (Fig. S4a–c) that are broadly anti-correlated with terrigenous input (sand fraction, mean sortable silt (mSS) and magnetic susceptibility (MS)) suggesting that part of the productivity signal is probably related to dilution by terrigenous material. However, the lack of a clear relationship between TOC/bOpal and δ18O diatom implies that the δ18O diatom signal is unlikely to be influenced or overprinted by local productivity changes. Thus, and consistent with previous work, we are confident that lower δ18O diatom values equate to increased glacial discharge.

Figure 2 Holocene glacial discharge, climatic proxies and forcing mechanisms. (a) δ18O diatom from core PS67/182-1 as a proxy for glacial melt with a 3 point moving average (blue; eastern Antarctic Peninsula (AP)). (b) δ18O diatom derived glacial discharge record from Palmer Deep, ODP Site 1098 A with a 3 point moving average (green; western AP)20. (c) δ18O diatom derived glacial discharge record from Adelie-George VI Land coast (black; EAIS glacial discharge)21. (d) SAM index with 70 year loess filter (red)18. (e) 100 year average deteurium reconstructed temperature anomalies from James Ross Island (JRI) ice core (yellow)23. (f) Subsurface ocean temperature (SOT) for eastern AP38. (g) TEX86 L SST derived temperature record from core Site 1098, Palmer Deep (peach)39. (h) El Junco Sand as an indicator of ENSO frequency (brown)43. (i) hyperspectral ratio (R850/R900) from a lake record on Macquarie Island (purple line), as proxy for Southern Hemisphere westerlies50. Lower values equate to stronger winds (thicker line is with 100-year interval second-order LOESS smoothing). (j) December and June insolation values at 60°S72. (k) Holocene reconstructions of the presence (dark blue bar) and absence (red bar) of AP ice shelves that have broken up or retreated significantly in the past few decades (modified from Hodgson6). Year of recent ice shelf collapses appears left of panel. All data are plotted against age (cal. yrs BP) with an additional A.D. age scale for the last 1000 years. Dashed/dotted vertical lines show significant change points in the δ18O diatom data (see Materials and Methods). Full size image

Drivers of glacial discharge during the past 6,250 years

6.25–1.62 ka

The consistently high δ18O diatom values prior to c. 1,617 cal. yr BP are indicative of a marine-water dominated signal with a reduced influence from glacier and iceberg melting relative to modern-day values (Fig. 2a). We attribute this period of low variability, relatively uniform glacial discharge to normal growth and calving of glaciers following retreat from LGM limits. Regional deglaciation was largely complete by the early Holocene in the Weddell Sea embayment34,35, and although local glacier-front variability has been observed in some areas36 the ice sheet appears to have reached a stable configuration by the early to mid-Holocene. Multi-proxy analysis of marine core JPC38 recovered at the northern end of Prince Gustav Channel (Fig. 1c) indicates that the eastern AP region experienced cooler conditions, glacier advance and sea-ice expansion between 5,000–1,900 cal. yrs BP37 while sub-surface ocean temperatures (SOT = 50–400 m water depth) shows centennial-scale warm events that are superimposed on a gradual warming trend (0.3 °C) from ~7,000 to 500 cal. yr BP (Fig. 2f)38. The JRI ice core shows an interval of stable climate persisted from, 9,200 to 2,500 yr BP23. Reduced glacial discharge20, increased sea-ice cover and a relatively cool sea surface temperatures (SST)39 were also observed on the western AP at this time, suggesting coherent temperature trends during the mid-Holocene (Fig. 2g). Cooler atmospheric conditions during this time interval have been linked to reduced solar insolation (Fig. 2j) which would have reduced glacier melt directly along both sides of the Peninsula. Barbara et al.37 further suggest that a northward displacement of the Antarctic Circumpolar Current (ACC) occurred at this time, reducing the entrainment of warm CDW into the Weddell Gyre resulting in sea-ice growth and reduced ice-shelf melting. The apparent increase in δ18O diatom variability between c. 3,200 and 1,900 cal. yr BP is broadly coincident with a mid-Holocene climate optimum inferred from multiple indicators of increased biological production in lakes on nearby Beak Island (c. 3,800–1,400 cal. yr)40 and on Signy Island (3,169 and 2,120 cal. yr BP)41. (Fig. 1c). It also corresponds with a possible, though poorly dated, period of ice-shelf absence in Prince Gustav Channel42 (Fig. 2k).

1.62–0.72 ka

Glacial discharge is more variable between 1,617 and 720 cal. yr BP (A.D. 333–1230) peaking around ~1,100 cal. yr BP before declining (Fig. 2a). This variability, however, is not identified in our change point analysis. Climate variability in the Peninsula region during this interval has been linked to a peak in summer insolation and an intensification of El Niño/Southern Oscillation (ENSO)43,44 (Fig. 2h). Along the western AP, the high SSTs between 1,600 and 500 cal. yr BP (Fig. 2g) have been attributed to La Niña events39, which are thought to have brought warm air from lower latitudes towards Antarctica via the westerlies12 and driven increased frontal melting of marine-terminating glaciers20. The lack of distinct correlation between glacial discharge at site PS67/182-1 and ENSO-proxies (Fig. 2a,h) throughout this interval suggests that ice masses on the eastern AP and Weddell Sea embayment responded non-linearly to climate forcing, or that the amplitude of forcing was insufficient to drive the large-scale increases in glacial discharge that is seen during subsequent stages (i.e., the last ~500 years). In this context, Mulvaney et al.23 have noted the development of opposing east-west temperature trends during the late Holocene (~1,500–550 cal. yrs BP), where warmer SST on the western AP coincides with cooler atmospheric temperatures on the eastern side. Opposing temperatures during this period have been related to the establishment of Antarctic dipole-like conditions, driven by opposite temperature and sea-ice anomalies in the Weddell and Amundsen-Bellingshausen seas. The development of dipole-like conditions during the late Holocene, which has been linked to ENSO, might explain why the long-term trend to increasing glacial discharge on the western AP, starting around ~2,500 cal. yr BP20 (Fig. 2b), is not replicated at site PS67/182–1. Limited ocean-driven melting is also supported by cooler, or at least stable, SOT inferred from core JPC38 (Fig. 2f)38. Recently, Charman et al.45 have argued against opposing temperature trends based on the analysis of moss banks on either side of the Peninsula. Either this suggests an incomplete understanding of west-east AP climate trends during this time period, or that individual proxies from marine/terrestrial archives are sensitive to different, or even multiple, climate variables.

0.72 to 0 ka

Change points at 720 cal. yr BP (A.D. 1230) and 244 cal. yr BP (A.D. 1706) bracket pronounced changes in discharge at site PS67/182-1, which first declines until ~550 cal. yr BP (A.D. 1400) before displaying a pronounced increasing trend until the modern day (A.D. 2005). At the same time, both the JRI ice core23 and the Beak Island lake record40 on the eastern AP show a warming trend from 550 and 543 cal. yrs BP respectively (Fig. 2e), while SOTs on the eastern AP appear to increase from ~500 cal. yr BP (Fig. 2f)38. Glacial discharge on western AP also shows a pronounced increase around 550 cal. yr BP, although this is limited to 4 data points and is also superimposed on a longer-term increasing trend that started much earlier (~2,500 cal. yr BP)20 (Fig. 2b). Warmer atmospheric temperatures on the eastern AP after ~550 cal. yr BP have been associated with variations in SAM which shows a significant switch - from its positive to negative phase between 1300 and 1460 cal. yr BP - followed by a trend to positive SAM since the mid-fifteenth century18,40 (Fig. 2d). We note that variations in glacial discharge at site PS67/182-1 also appear to follow steps in the SAM, with low but increasing discharge from ~550 cal. yr BP (A.D. 1400) until 244 cal. yr BP (A.D. 1706) associated with a predominantly –SAM, but increasing to its positive phase thereafter (Fig. 3a,b). After 244 cal. yr BP (A.D. 1706), glacial discharge increases rapidly to a level that is unprecedented during the past 6,250 years, coincident with predominantly + SAM as well as increasing El Niño conditions (Figs 2a, 3 and S2). We therefore argue that + SAM acted as an important driver of enhanced glacial discharge along the eastern AP, particularly after A.D. 1706.

Figure 3 Climate evolution during the past 1000 years. (a) Glacial melt from the δ18O diatom index of PS67/182-1 with a 3 point moving average (blue). (b) The Southern Annular Mode (SAM) with a 70 year loess filter (red)18. (c) Temperature anomalies from JRI ice core with a 3 point moving average (yellow)23. (d) Multiproxy reconstruction of Niño3.4 sea surface temperature (SST) with a 3 point moving average (dark red line)73. (e) δ18O diatom derived glacial discharge record from Adelie-George VI Land coast (black)21. (f) δ18O diatom derived glacial discharge record from Palmer Deep, ODP Site 1098 A with a 3 point moving average (green)20. (g) hyperspectral ratio (R850/R900) from a lake record on Macquarie Island (purple), as proxy for Southern Hemisphere westerlies50. Lower values equate to stronger winds (thicker line is with 100-year interval second-order LOESS smoothing). The upper part of this record (A.D. 1903 to present; denoted by a black line) is potentially disturbed by erosion associated with rabbit activity50. Dashed/dotted vertical lines show significant change points in the δ18O diatom data (see Materials and Methods). Full size image

SAM as a driver of glacial discharge during the late Holocene

SAM denotes the pressure gradient between mid and high latitudes8 and, during a more positive SAM, the belt of westerlies intensifies and contracts poleward, bringing stronger, warmer westerlies to the AP region. Analysis of contemporary temperature data (spanning 1979–2015) reveals that SAM impacts the west and eastern sides of the AP in different ways12. On the eastern AP, inter-annual variability of temperatures is most sensitive to zonal wind (west-east) anomalies crossing the Peninsula and resultant leeside adiabatic warming (Föhn winds) rather than to meridional wind anomalies (i.e., north-south variations), which is closely tied to variability in the zonal portion of the SAM pattern12. More frequent Föhn events drive enhanced surface melting that has been linked to collapse of the Larsen B Ice Shelf in 2002 through hydrofracture14,46. The oceanic response to the shifting SAM – on the eastern side of the AP - is mostly through Ekman transport16. Positive SAM is associated with increased wind stress and reduced sea-ice47,48 and, over longer time periods, stronger westerlies promote greater entrainment of warm CDW water into the Weddell Gyre16 with the potential to increase melting of fringing ice shelves38.

We hypothesise that a progressive shift to positive SAM since the mid-fifteenth century, and particularly since the early 1700s promoted stronger westerlies that triggered increases in glacial discharge, via the mechanism that we see today; that is, atmospheric and ocean-driven thinning and retreat of ice shelves. Whilst there is a dearth of wind-proxies for the AP covering this time-interval, sea salt aerosol (ssNa+) concentrations in EPICA Dome C ice core49 (Figs S1 and S2) indicate increasing and decreasing wind in phase with SAM from ~500 cal. yr BP (Fig. S2). In addition, wind proxy records from sub-Antarctic Macquarie Island50 (Fig. 3g) and southernmost Patagonia51 suggest a significant poleward displacement and likely intensification of westerlies ~A.D. 1700, when glacial discharge at site PS67/182-1 increases beyond the long-term mean (Fig. 3). At the same time, atmospheric and ocean warming on the eastern AP has been observed from ~550–500 cal. yr BP, increasing in progressive steps towards the present day (Fig. 2e,f)23,38. The apparent down turn in SOT around A.D. 2000 (Fig. 2f) has been described in terms of intensified downwelling linked to the well-publicized phase of atmospheric cooling of the Peninsula region38. Although speculative, we also note that enhanced glacial discharge has been observed along the Adélie-George V Land coast (Figs 1c and 3e) from ~A.D. 170021 which could imply a circum-Antarctic response to SAM/wind forcing. Our explanation is consistent with observations linking + SAM with the recent retreat of outlet glaciers along the Pacific coastline of EAIS, either through rising air temperatures or increased coastal upwelling of CDW52, although is at odds with the original interpretation linking the decrease in δ18O diatom to intensified ENSO21. Irrespective of the driver, it would appear that at least three sectors of the ice sheet (western and eastern APIS, EAIS) have undergone enhanced mass loss during the past ~300–500 years. In the case of the eastern AP, the most recent changes (since A.D. 1706) are unprecedented during the past 6,250 years.

The mechanisms driving SAM trends during the last millennia are only partially understood. Coupled climate simulations indicate that increasing solar irradiance after the Spörer Minimum (~A.D. 1415–1534) resulted in a small positive forcing on SAM10,18. Abram et al.18 also note a strong co-dependence between ENSO and SAM during the last 1000 years, suggesting the maximum in Niño3.4 SST during the fifteenth century might have contributed to the SAM minimum (Fig. 3b,d). Instrumental studies indicate that ENSO variability in the tropical Pacific interacts with storm tracks in the south Pacific via a Rossby wave train, whereby El Niño (La Niña) events drive cool (warm) conditions on the AP and are associated with negative (positive) SAM states53. This is seen as an inverse relationship between the Niño3.4 reconstruction and the JRI temperature record, lending support to the idea that El Niño and negative SAM was a persistent feature during the late Holocene18. El Niño is thought to become a secondary influence on SAM during the twentieth century when a positive trend in SSTs (El Niño) (Fig. 3d) would be expected to impose negative forcing on the mean state of SAM18. However, SAM shows a continuing trend to a more positive state (manifest as greater glacial discharge at our site) which is thought to reflect the dominant influence of rising greenhouse gas levels and ozone depletion. However, despite evidence for a clear co-dependence between SAM and ENSO during the late Holocene, questions exists about the relative impacts on AP temperatures, and how this might vary on the west and east sides of the Peninsula.

Summary and outlook

We have shown that glacial discharge along the eastern AP in the NW Weddell Sea embayment has varied throughout the past 6,250 years (Fig. 2a). Initially long-wavelength mechanisms such as orbital forcing and ENSO frequency provide plausible mechanisms for these changes. An increasing trend in glacial discharge is seen from ~A.D. 1400 (550 cal. yr BP), although it wasn’t until A.D. 1706 (244 cal. yrs BP) that discharge at site PS67/182-1 exceeded the level experienced during the past 6,250 years. Comparison with available proxy data indicates that increases in glacial discharge appear to have been driven, at least in part, by a positive phase of the SAM and intensification of westerly winds (Figs 3 and S2), acting on both the atmosphere and ocean to drive thinning of fringing ice shelves and marine-terminating glaciers. If our interpretation is correct, one possible implication of our data is that the early onset of melting (after A.D. 1706) has contributed to the near synchronous loss of ice shelves along the eastern AP6,7 - most notably the Larsen A-Prince Gustav Channel in 1995 and Larsen B in 2002 (Fig. 2k) - providing further support to the idea that ice shelves on the eastern AP have been predisposed to collapse by hundreds, and in some cases thousands54 of years of thinning. This contrasts with the irregular timing of Holocene ice shelf collapses known to date (Fig. 2k). Indeed, no single forcing mechanism has been identified as providing a tipping point for past ice shelf retreat6 which could explain why the signal of past collapses in our δ18O diatom record is more diffuse than the pronounced changes witnessed in recent decades.

Models predict further poleward intensification of the westerlies11,18 and continued greenhouse warming around Antarctica55. Given the sensitivity of the APIS (and Antarctic Ice Sheet more generally) to variations in the westerlies, it is likely that mass loss will continue to accelerate through the mid-late twenty first-century, raising global sea-level1, impacting marine ecosystems3 and potentially leading to meltwater-induced sub-surface ocean warming which has the potential to drive even greater melting underneath ice shelves4.