Little Ice Age effects on Southern Ocean thermohaline circulation

doi:10.21203/rs.3.rs-3779321/v1

Download PDF

Article

Little Ice Age effects on Southern Ocean thermohaline circulation

https://doi.org/10.21203/rs.3.rs-3779321/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

The Little Ice Age (LIA) was one of the strongest climate perturbations in the last millennium, with widespread albeit asynchronous periods of cooling globally¹. Its effects on oceanic properties and thermohaline circulation have been questioned, however². We provide evidence of changes in the properties of Antarctic Intermediate Water (AAIW) over the last millennium that are synchronous with LIA cooling events and that appear to derive proximately from variations in the proportion of Upper Circumpolar Deep Water (UCDW) in the AAIW source mixture, and ultimately from changes in the strength and position of the zonal west winds over the Southern Ocean (SO). Export of AAIW from the SO influences climate and biogeochemical cycles globally by transporting heat, freshwater, nutrients and carbon to lower latitudes^3-5, implying widespread impacts of its LIA-correlated variability. AAIW tracking of the LIA ceased in the late 1800s, which we suggest marked the onset of over-riding impacts of anthropogenic climate change on SO circulation.

Earth and environmental sciences/Ocean sciences/Physical oceanography

Earth and environmental sciences/Climate sciences/Palaeoceanography

Southern Annular Mode

Temperature

Salinity

Bamboo coral

Reservoir age

Oxygen isotopes

Antarctic Intermediate Water

Circumpolar Deep Water

Southern Ocean overturning circulation

Our primary data source is the skeletal composition of long-lived calcitic deep-water corals (bamboo corals - Keratoisididae), supplemented by information on the historical growth rates of AAIW-resident fish. Radiocarbon and U-series dating⁶ indicate individual bamboo coral colonies live for up to 600 years, potentially providing in each colony a long-term environmental record. We assembled, dated and analysed geochemical records (see Methods) from fifteen colonies collected in or near the core depth of AAIW (800-1200 m) from the sub-tropical and temperate SW Pacific (Suppl. Table 1). Temperature, salinity and pH measured close to the seamount surfaces from which the corals were collected match those measured 10s of kilometres from them⁷, indicating that environmental conditions experienced by the corals and recorded in their skeletons are representative of water masses over a broader region.

The SW Pacific coral records (Fig. 1) show that at the most intensively sampled site (seamounts off Tasmania, Australia), magnesium/calcium (Mg/Ca) ratios from nine corals are coherent once aligned on the basis of deposition date (which ranges from 700 AD to modern (2008)). Records from two other sites (the Chatham Rise [NZ] and the Norfolk Ridge) show the same long-term trends as the Tasmanian time series. Mg/Ca ratios were consistently high until ca. 1450, declined over the next century, increased in the mid-to-late 17^th century to almost the earlier levels, then declined again to trough in the mid-1800s before rising slightly and then slowly declining from about 1900 onwards. These long-term trends are also evident in isotopic records obtained from a sub-set of the Tasmanian samples (Fig. 2). Oxygen and carbon isotope ratios (δ¹⁸O and δ¹⁴C) and reservoir ages were measured in three corals that spanned the last ~850 years and boron isotope ratios (δ¹¹B) in a specimen (K2) that spanned the last ~400 years. In all three corals, δ¹⁸O correlates positively with Mg/Ca (Fig. 2), after adjusting for ‘vital effects” consequent of the calcification process and the effect of temperature change on isotope ratios (see Methods). The decline, starting in the late 1400s, and subsequent undulations in δ¹⁸O closely parallel those of Mg/Ca. The δ¹⁸O time series are congruent for the two specimens collected 10s of kms apart (K7 and K2), while values are slightly elevated in the specimen collected further south (K6). The δ¹¹B record of K2 co-varies antithetically with Mg/Ca and δ¹⁸O, rising as Mg/Ca ratios fell between 1700 and 1900 and then stabilizing over the last ~100 years (Fig. 2). Reservoir ages, quantified as the difference between δ¹⁴C and “wiggle map” ages, are stationary early in the data series (see Methods). In the youngest specimen, K2, however, they rise initially, but subsequently decline from ~1700 to 1975 (F_1,31 = 24.5, p < 0.0001) (Fig. 2). Reservoir ages in K2 correlate negatively with δ¹¹B (F_1,30 = 4.35, p = 0.046), but only weakly track Mg/Ca (F_1,38 = 2.78, p = 0.10) and adjusted δ¹⁸O (F_1,38 = 1.68, p = 0.20).

Mg/Ca ratios are temperature-dependent in a wide range of biogenic and inorganic calcites⁸. In gorgonian calcite, specimen mean Mg/Ca ratios have been suggested as proxies for ambient temperature, either directly^9-12 or indirectly as a result of temperature-dependent growth rates^10,13. Whether directly or indirectly, from a global calibration of Mg/Ca in keratoisidid corals (see Methods), we infer that AAIW in the SW Pacific has cooled by ~1.0 – 3.7 Cº (95% CI) since the mid-15^th century (Fig. 1). The inferred cooling in the industrial era (post 1900, see below) is consistent in magnitude with recent instrumental records^14,15 (see Methods) and with declining temperature-correlated growth rates of AAIW-resident fishes on South Pacific seamounts (Suppl. Fig. 1). Although large relative to recent changes in the instrumental record, the implied multi-century decline in temperature is within the range suggested for AAIW over the Holocene¹⁶. Oxygen isotope ratios in seawater reflect ambient temperature, salinity and water mass characteristics¹⁷. As vital and temperature effects were removed from our analysis, the δ¹⁸O record likely indicates freshening of AAIW¹⁸ since the mid-15^th century. δ¹¹B is pH-dependent in corals. While skeletal aragonite δ¹¹B represents calcifying fluid pH in scleractinians, due to their ability to upregulate pH^19,20, gorgonian calcite δ¹¹B largely matches ambient seawater pH ^{21, see, however, 22}. Accordingly, the K2 δ¹¹B record suggests a sustained increase in AAIW pH since the early 1700s. Over that same period, reservoir ages decline by ~110 years. Taken together, the proxy records indicate that AAIW has generally cooled and freshened since ~1450 and, since at least ~1750, has also increased in pH and declined in reservoir age.

The inference of multi-century cooling and freshening at intermediate depths over a broad region of the SW Pacific suggests a response to large-scale climate drivers. Similarities between the coral records and regional and global climate reconstructions support this hypothesis (Fig. 3). The multi-century decline in Mg/Ca ratios prior to ~1850, and the troughs in the mid-16^th and 19^th century, are also seen in reconstructions of air temperature on the Weddell Sea coast²³ (Fig. 3B) and in southern South America²⁴ (Fig. 3C), in the SAM index²⁵ (Fig. 3D), and in the LIA-correlated troughs in surface air temperature reconstructed from Greenland ice cores²⁶ (Fig. 3E). LIA conditions varied regionally, but were generally characterised by cool conditions globally, with one or more embedded warm periods¹. This is consistent with our observations, as well as with reconstructed Antarctic terrestrial, cryospheric and oceanic conditions^{e.g., 27,28}. Cooling associated with the LIA coincides with equatorward expansion of the westerly winds (a negative index of the Southern Annular Mode [SAM]), and presumably weaker winds, between 1500 and 1650 and in the mid-1800s (Fig. 3D), shifts also suggested by analysis of Southern Hemisphere tree rings (Suppl. Fig. 2).

Weaker and equatorward-shifted westerly winds likely contributed to cooler and fresher AAIW. The upper cell of the SO overturning circulation consists of poleward flow of UCDW balanced by equatorward flow of intermediate waters formed in the SO (AAIW and Sub-Antarctic Mode Water [SAMW])^29,30. In this sense, AAIW is ultimately derived from CDW: wind-driven upwelling carries old CDW to the surface mixed layer where it is transformed to lighter, cooler, fresher and well-ventilated Antarctic Surface Water (AASW) by air-sea fluxes and addition of sea ice melt^31,32; AASW is then subducted into the ocean interior where its properties are further modified by isopycnal and diapycnal mixing to form AAIW³³. Therefore, many physical mechanisms might explain a link between changes in AAIW properties and large-scale climate drivers, including changes in air-sea exchange, wind- and buoyancy-driven circulation, eddy fluxes, turbulent mixing, and the volume of UCDW in the AAIW mixture. As a consequence, it is difficult to identify the drivers of the inferred changes in AAIW from temperature and salinity alone. However, while the temperature, salinity, and oxygen content of UCDW can be rapidly reset by air-sea interaction once it is entrained in the surface mixed layer, the equilibration times for CO₂ (~1 year) and Δ¹⁴C (~10 years) are long relative to the residence time of SO surface waters³⁴. Therefore, the signature of upwelled old UCDW persists in SO surface waters, which are lower in pH and older (more negative Δ¹⁴C) than other surface waters in the global ocean³⁴. AAIW thus retains a signature of the UCDW from which it is derived. Less upwelling and/or entrainment of old, relatively saline, and carbon- and nutrient-rich UCDW would produce AAIW that is fresher, higher in pH, lower in nutrients, and with a lower reservoir age. The correlated, inferred increase in pH and reduction in reservoir age in AAIW after ~1750 therefore suggests a gradual reduction in the volume of UCDW incorporated into AAIW. The increase in reservoir age between the early 1600s and ~1750, during a period when temperature and salinity of AAIW increased to a peak and then declined, is consistent with greater input of UCDW during that time period. Nutrients supplied by upwelled UCDW are not fully utilised in the surface layer of the Southern Ocean before being subducted, so less upwelling of UCDW would also result in less export to lower latitudes in AAIW. Inferred silicate levels (Ba/Ca proxy)^{10, 35}in SW Pacific AAIW, as predicted by this hypothesis, decline significantly from the late 1600s until the industrial era (Fig. 4). Nitrate availability in the central Pacific, as inferred from δ¹⁵N in Hawaiian corals, has declined since 1350³⁶ in parallel with the inferred cooling and freshening of AAIW (lagged 30 years to account for transit time to the Pacific) (Suppl. Fig. 4). Less UCDW in the AAIW mixture can also account for reduced Southern Hemisphere atmospheric radiocarbon ages in the mid-to-late 1800s³⁷ and a long-term decline in oceanic CO₂ flux since ~1700³⁸.

Less UCDW in the AAIW mixture could result from weaker upwelling of UCDW or by a reduction in the entrainment of UCDW into the AAIW by vertical mixing in the ocean interior³⁹. Weaker upwelling is consistent with a shift to weaker and equatorward-shifted winds (i.e., negative SAM), as inferred to have occurred during the LIA in general, and in the mid-16^th and 19^th centuries in particular²⁵. A strengthening and poleward shift of the westerly winds (i.e., positive SAM), on the other hand, results in stronger upwelling of UCDW⁴⁰. Decadal changes in strength of the upper overturning circulation inferred from ocean tracer observations^41,42confirm a link between UCDW upwelling and the strength and latitude of the westerlies, despite the tendency for change in wind-driven overturning to be partly compensated by changes in eddy transport⁴³. Weaker winds characteristic of the LIA would also result in less turbulent entrainment of UCDW across the base of the surface mixed layer.

The proxies of atmospheric conditions in Fig. 3 all reach a minimum in the mid-1800s and then rise until the end of the records. The coral Mg/Ca, in contrast, remains low (Fig. 4). This divergence of the proxy records suggests different physical mechanisms at play after the late-1800s. The increase in the SAM index after the mid-1800s suggests a shift to stronger and more poleward westerly winds, consistent with warming of coastal air temperatures in the Weddell Sea and southern South America, and coincident with warming of Greenland air temperatures at the end of the LIA (Fig. 3). Based on the relationships observed prior to the late 1800s, we would expect poleward-shifted winds (positive SAM) to be associated with high Mg/Ca (warm AAIW) and stronger upwelling of UCDW. The Ba/Ca proxy suggests silicate levels increased in the late 1800s⁴⁷ (Fig. 4), also consistent with stronger upwelling of UCDW. However, AAIW temperatures remained low. The proxy evidence therefore suggests that upwelling of UCDW strengthened in the late 1800s but that surface processes (e.g. air-sea exchange or sea ice formation and melt, and their influence on near-surface stratification) vented the heat supplied by UCDW, keeping surface (and therefore AAIW) temperatures low. We speculate that the late-1800s change in the relationship between AAIW temperature and UCDW upwelling proxies reflects the onset of the impacts of anthropogenic forcing on the Southern Ocean. The late 1800s onset in the SO, as opposed to early-to-mid 1800s in the Northern Hemisphere and tropical oceans, also corroborates a half-century lag in industrial impacts between the Northern and Southern Hemisphere climates⁴⁸, the cause of which remains uncertain.

Changes in AAIW properties and circulation impact the strength of the ocean carbon sink^4,5, ventilation of the subtropical thermocline⁴⁴, ocean primary productivity³, the biology of deep-sea taxa⁴⁵, and ocean circulation in regions as distant as the North Atlantic⁴⁶. Consequently, LIA-correlated changes in AAIW properties could have broad historical consequences for the global ocean. The multi-century coral record also provides context for recent observations and models of change in AAIW properties. Changes in the properties of SO intermediate waters in the instrumental record have been attributed to a variety of drivers, including shoaling or deepening of isopycnals (Suppl. Fig. 4), changes in air-sea exchange of heat and freshwater (i.e. evaporation and precipitation), in sea ice formation and melt, and wind-driven changes in ocean circulation. Our data show that AAIW properties broadly track the LIA through to the end of the 19^th century, but then diverge from it (Fig. 4). Given the sparse historical oceanographic record for the SO and its key role in global climate, our data fill an important gap in developing accurate historical models for the SO.

Acknowledgements and Funding Sources

We thank J. Adkins, P. Alderslade, S. Cairns, T. Correge, R. Gurney, J.A. Koslow, E. Lazlo-wasem, C. MacRae, P. Marriott, M. McCulloch, C. Proctor, B. Richer de Forges, A. Thresher, A. Williams, N. Wilson and R. Wood for provision and identification of specimens, assistance in analyses, and advice during the course of this project, B. Sloyan and H. Bostock for discussions about the significance of the results, the crews of the RV Southern Surveyor, RV Thomas G. Thompson and the ROV Jason2 for their professional assistance in field sampling, and our colleagues in CSIRO and the Australian and the New Zealand fishing industries for specimens. Components of this work were supported by the Australian Commonwealth Environmental Research Fund, the Australian Department of Environment, Water, Heritage, and the Arts, the Australian Greenhouse Office, the Australian National Climate Adaptations Research Program, the CSIRO Wealth from Oceans and Climate Adaptation Flagships, the National Science Foundation, and a grant of ship time by the Australian National Research Facility.

Author contributions

RET, SJF, SRdeF, HN and JAT collected and analyzed data, RET, SRR and JAT wrote the ms, and RET, SRdeF and DMT obtained specimens.

Competing interests

The authors declare no competing interests.

The aging and geochemical data used in this study will be permanently archived in the CSIRO Data Access Portal (https://data.csiro/collections/) upon acceptance.

Jones PD, Briffa KR (2001) The “Little Ice Age”: local and global perspectives. Climate Change, 48:5-8.
Keigwin LD, Boyle EA (2000) Detecting Holocene changes in thermohaline circulation. Proc Nat Acad Sci USA, 97: 1343-1346.
Sarmiento J L, Hughes TMC, Stouffer RJ, Manabe S (2004) High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature 427:56-60.
Khatiwala S, Primeau F, Hall T (2009) Reconstruction of the history of anthropogenic CO₂ concentrations in the ocean. Nature 462:346–349.
Fröhlicher TL, Sarmiento, JL, Paynter, DJ, et al. (2014) Dominance of the Southern Ocean in anthropogenic carbon and heat uptake in CMIP5 models. J Clim 28:862-886.
Roark EB, Guilderson, TP, Flood-Page, S, et al. (2005) Radiocarbon-based ages and growth rates of bamboo corals from the Gulf of Alaska. Geophys Res Lett 32:L04606.
Thresher RE, Wilson NC, MacRae CM, Neil H (2010) Temperature effects on the calcite skeletal composition of deep-water gorgonians (Isididae). Geochim Cosmochim Acta, 74:4655-4670.
Mucci A (1987) Influence of temperature on the composition of magnesium calcite overgrowths precipitated from seawater. Geochim Cosmoschim Acta 51:1977-1984.
Weinbauer MG, Brandstatter F, Velimirov B (2000) On the potential use of magnesium and strontium concentrations as ecological indicators in the calcite skeleton of the red coral (Corallium rubrum). Mar Biol 137:801-809.
Thresher RE, Fallon S, Townsend AT (2016) A “core-top” screen for trace element proxies of environmental conditions and growth rates in the calcite skeletons of bamboo corals (Isididae). Geochim Cosmochim Acta 193:75-99.
Chaabane S, Correa, ML, Ziveri, P, et al. (2019) Elemental systematics of the calcitic skeleton of Corallium rubrum and implications for the Mg/Ca temperature proxy. Chem Geol 254:237-258.
Flöter S, Fietzke, J, Gutjahr, M, et al. (2019) The influence of skeletal micro-structures on potential proxy records in a bamboo coral. Geochim Cosmochim Acta 248:43-60.
Thresher RE (2009) Environmental and compositional correlates of growth rate in deep-sea bamboo corals (Gorgonacea; Isididae). Mar Ecol Prog Ser 397:187-196.
Bindoff NL, McDougall TJ (2000) Decadal changes along an Indian Ocean section at 32ºS and their interpretation. J Phys Oceanogr 30:1207–1222.
Johnson GC, Orsi AH (1997) Southwest Pacific Ocean Water-mass changes between 1968/69 and 1990/91. J Clim 10:306-316.
Poggemann, D.-W., Nurnberg, D., Hathorne, S.C., Frank, M., Rath, W., Reissig, S., & Bahr, A. (2018). Deglacial heat uptake by the Southern Ocean and rapid northward redistribution via Antarctic Intermediate Water. Paleocean Paleoclim, 33, 1292-1305.
Druffel ERM (1997) Geochemistry of corals: proxies of past ocean chemistry, ocean circulation and climate. Proc Nat Acad Sci USA 94:8354–8361.
Thresher RE, Neil H (2016) Scale dependence of environmental and physiological correlates of d¹⁸O and d¹³C in the magnesium calcite skeletons of bamboo corals (Gorgonacea; Isididae). Geochim Cosmochim Acta 187:260-278.
McCulloch MT, Trotter, J, Montagna, P, et al. (2012) Resilience of cold-water scleractinian corals to ocean acidification: boron isotope systematics of pH and saturation up-regulation. Geochim Cosmochim Acta 87:21–34.
Anagnostou E, Huang K-F, You C-F, Sikes EL, Sherrell RM (2012) Evaluation of boron isotope ratio as a pH proxy in the deep sea coral Desmophyllum dianthus: evidence of physiological pH adjustment. Earth Planet Sci Lett 349-350:251-260.
Le Goff C, Tambutte E, Venn AA, Techer N, Allemand D, Tambutte S (2017) In vivo pH measurement at the site of calcification in an octocoral. Nature Sci Rpts 7:11210; doi:10.1038/s41598-017-10348-4.
Farmer JR, Hönisch B, Robinson LF, Hill TM (2015) Effects of seawater-pH and biomineralization on the boron isotopic composition of deep-sea bamboo corals. Geochim Cosmochim Acta 155:86-106.
Stenni B, Curran, MAJ, Abram, NJ, et al. (2017) Antarctic climate variability on regional and continental scales over the last 2000 years. Clim Past 13:160-9-1634.
Neukom R, Luterbacher, J, Villalba, R, et al. (2011) Multiproxy summer and winter surface air temperature field reconstructions for southern South America covering the past centuries. Clim Dyn 37:35-51.
King J, Anchukaitis KJ, Allen K, Vance T, Hessl A (2023) Trends and variability in the Southern Annular Mode over the Common Era. Nature Comm 14:2324; doi.org/10.1038/s41467-023-37643-1.
Dahl-Jensen D, Mosegaard K, Gundestrup N, Clow GD, Johnsen SJ, Hansen AW, Balling N (1998) Past temperatures directly from the Greenland ice sheet. Science 282:268-271.
Kreutz, KJ, Mayewski PA, Meeker LD, Twickler MS, Whitlow SI, Pittawala II (1997) Bipolar changes in atmospheric circulation during the Little Ice Age. Science 277:1294-1296.
Broecker WS, Sutherland SW. (2000) Distribution of carbonate ion in the deep ocean: support for a post-Little Ice Age change in Southern Ocean ventilation? Geochem. Geophys. Geosyst. 1: 1023, doi:10.1029/2000GC000039.
Sloyan, BM, Rintoul, SR. (2001) The Southern Ocean Limb of the Global Deep Overturning Circulation. J. Phys. Oceanogr., 31, 143–173, https://doi.org/10.1175/1520-0485(2001)031<0143:TSOLOT>2.0.CO;2.
Talley LD (2013) Closure of the global overturning circulation through the Indian, Pacific and Southern Oceans: schematics and transports. Oceanography 26:80-97.
Haumann FA, Gruber N, Munnich M, Frenger I, Kern S (2016) Sea-ice transport driving Southern Ocean salinity and its recent trends. Nature 537:89-92.
Abernathey, R, Cerovecki, I, Holland, P et al. (2016) Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning. Nature Geosci9, 596–601. https://doi.org/10.1038/ngeo2749
Li, Z, Groeskamp, S., Cerovecki, I, England, MH. (2022) The origin and fate of Antarctic Intermediate Water in the Southern Ocean. J Phys Oceanogr 52: 2873-2890. https://doi.org/10.1175/JPO-D-21-0221.1
Key, RM, Kozyr, A, Sabine, CL, Lee, K, Wanninkhof, R, Bullister, JL, Feely, RA, Millero, FJ, Mordy, C, Peng, T-H (2004) A global ocean carbon climatology: Results from Global Data Analysis Project (GLODAP). Global Biogeochem Cycles 18, GB4031, doi:10.1029/2004GB002247.
LaVigne, M, Hill, TM, Spero, HJ, et al. (2011). Calibration of a new deep ocean refractory nutrient proxy. Earth Planet Sci Lett 312: 506-515. http://doi.org/10.1016/j.epsl.2011.10.013.
Sherwood O.A., Guilderson TP, Batista FC, Schiff JT, McCarthy MD (2013) Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age. Nature 505:78-81.
Turney, C.S.M., Palmer, J., Hogg, A., et al. (2016) Multidecadal variations in Southern Hemispheric atmospheric ¹⁴C: evidence against a Southern Ocean sink at the end of the Little Ice Age CO₂ anomaly. Global Biogeochem Cycles, 30: 211-218.
Rubino, M, Etheridge, DM, Trudinger, CM, Allison, CE, Rayner, PJ, Enting, I, Mulvaney, R, Steele, LP, Langenfelds, RL, Sturges, WT. Curran, MAJ, Smith, AM (2016) Low atmospheric CO₂ levels during the Little Ice Age due to cooling-induced terrestrial uptake. Nature Geosci 9: 691-694.
Xue, L, Cai, WJ, Takahashi, T et al. (2018) Climatic modulation of surface acidification rates through summertime wind forcing in the Southern Ocean. Nature Comm 9: 3240. https://doi.org/10.1038/s41467-018-05443-7
Hall A, Visbeck M (2002) Synchronous variability in the Southern Hemisphere atmosphere, sea ice, and ocean resulting from the annular mode. J Clim 15:3043-3057.
Waugh, DW, Primeau, F, Devries, T, Holzer, M. (2013) Recent Changes in the Ventilation of the Southern Oceans. Science 339: 568-570. DOI:10.1126/science.1225411
Devries, T, Holzer, M, Primeau, F. (2017) Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning. Nature 542: 215–218. https://doi.org/10.1038/nature21068
Morrison, AK, Hogg, A McC (2013) On the Relationship between Southern Ocean Overturning and ACC Transport. J. Phys. Oceanogr. 43: 140–148. https://doi.org/10.1175/JPO-D-12-057.1.
Sallee J-B, Speer K, Rintoul S, Wijffels S (2010) Southern Ocean thermocline ventilation. J Phys Oceanogr 40:509-529.
Thresher RE, Morrongiello, J, Sloyan, BM, et al. (2014) Parallel decadal variability of inferred water temperatures for Northern and Southern Hemisphere intermediate water masses. Geophys Res Lett 41:1232-1237.
Graham JA, Stevens DP, Heywood KJ, Wang Z (2011) North Atlantic climate responses to perturbations in Antarctic Intermediate Water. Clim Dyn 37:297-311.
Thresher RE, Fallon SJ (2021) Apparent periodic and long-term changes in AAIW and UCDW properties at fixed depths in the southwest Pacific, with indications of a regime shift in the 1930s. Geophys Res Lett 48, e2020GL092329.
Abram NJ, McGregor HV, Tierney JE, Evans MN, McKay NP, Kaufman DS and the PAGES 2K Consortium (2016) Early onset of industrial-era warming across the oceans and continents. Nature 536: 411-418.

Dating

Corals were collected by dredging, in which case sample depth was determined as the mid-point of the targeted depth range, or by ROV (see ref. 1 for details), in which case depth was precisely known. Electron probe microanalyses follow procedures in refs. 2 a; oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) based on ref. 3 and boron isotope ratios (δ¹¹B) based on ref. 4. The analyses were each done at different laboratories (EPMA – CSIRO Mineral Products; O and C isotopes – NIWA; B isotopes – Univ. Western Australia), using different, but adjacent sections cut from the trunks of the corals. For the Australian and New Zealand samples, the Mg/Ca compositions of the Norfolk Ridge specimens were analysed using ICP-AES⁵. Dating methods varied among specimens, reflecting the range of studies involved. Most specimens were dated using by radiocarbon analysis of calcite, done at the Australian National University, following procedures in refs. 6 and 7 and again using adjacent coral sections, extrapolated to full age. Reservoir ages for deep-water in the SW Pacific poorly known. For analysis of the Tasmanian material, we used 970 years, based on ref. 8 and on the alignment between the growth rates of co-located known age orange roughy and coral Mg/Ca (Suppl. Fig. 1). For the Norfolk Ridge specimens, dates were assigned based on differences in calcite radiocarbon ages relative to a modern specimen (DW1697). L11, K9 and K11 were dated by radiocarbon analysis of surface-derived organic tissue in nodes^7,9. L11 and K9 were dated based on the “bomb” radiocarbon signal. For K11, we assumed a surface reservoir age of 330 years, based on ref. 10. Z10909, live caught, was dated by increment count¹¹ and K19 by “wiggle mapping” the Mg/Ca series of the apparently long dead (surface pitted and eroded) specimen onto that of a radiocarbon dated specimen (K18) from the same area. We also note that for the corals overall, the correspondence between the millennial Mg/Ca trajectory and regional climate variables, discussed in the text, corroborates our age estimates for the corals.

Calibration of proxies

We estimate the amount of isobaric temperature change in SW Pacific AAIW based on a global calibration between modern ambient temperature and specimen-mean Mg/Ca ratios for 71 keratoisidid corals (0.14 Cº per mmol/mol [95% CI 0.09 – 0.21], p < 0.0001) (Suppl. Fig. 5A). The slope is similar for 33 SW Pacific corals alone (m = 0.12, CI 0.03 – 0.20, p < 0.02) and for 38 specimens from all other sites pooled (m = 0.15, CI 0.07 – 0.24, p < 0.001), suggesting it is robust. The slopes are much less than those reported for pooled data spanning what is now four gorgonian families¹², but previously all considered isidids, reflecting differences among the current families in ambient temperature ranges and apparent temperature sensitivity of Mg/Ca ratios. The slopes of correlations within specimens at seasonal and interannual scales are highly variable and may differ among sites/thermal regimes, possibly due to effects of different growth rates¹³. However, the overlapping and concordant trends for the SW Pacific specimens suggest a common direct or indirect response to temperature change. In that regard, over the industrial era, Mg/Ca is highly variable, but trends downwards in all specimens (Suppl. Fig. 5B), which is consistent with reports of cooling of AAIW on isobaric surfaces over the last half century. The reported magnitude of the decline varies geographically, but near the sampled area ranges from ~0.005 to 0.02º C/yr^14-17, which is similar to our pooled estimate of ~0.005º (CI ~0.003 – ~0.007) C/yr since ~1900 (Suppl. Fig. 5B).

Changes in salinity are estimated based on a variant of the lines method¹⁸, detailed by ref. 3. Adjusting the residual δ¹⁸O time series for temperature effects steepened and increased the significance of the decline across the radius of K2 (F_1,217 = 330.2, p< 0.001), but the decline was significant without the temperature adjustment (F_1,217 = 8.5, p< 0.005).

References

1. Thresher RE, et al. (2014). Strong depth-related zonation of megabenthos on a rocky continental margin (~700-4000 m) off Southern Tasmania, Australia. PLoS One 9(1): e85872; doi:10.1371/journal.pone.0085872.

2. Gunn J, Harrowfield I, Proctor CH, Thresher RE (1992) Wavelength dispersive electron probe microanalysis of calcified tissues in fishes - analysis of techniques appropriate to studies of age and stock discrimination. J Exp Mar Biol Ecol 158:1–36.

3. Thresher RE, Neil H (2016) Scale dependence of environmental and physiological correlates of d18O and d13C in the magnesium calcite skeletons of bamboo corals (Gorgonacea; Isididae). Geochim Cosmochim Acta 187:260-278.

4. McCulloch MT, Holcomb M, Rankenburg K, Trotter JA (2014) Rapid, high-precision measurements of boron isotopic compositions in marine carbonates. Rapid Comm Mass Spectrom 2704–2712; doi:10.1002/rcm.7065.

5. Richer de Forges S (2001) Analyses des paleotemperatures mesurees sur des squelettes de gorgones isidae de profondeur. Rapport de stage de DEUG: IRD/Noumea (maitre de stage Thierry Correge):11 pp.

6. Fallon SJ, Fifield LK, Chapell JM (2010) The next chapter in radiocarbon dating at the Australian National University: status report on the single stage AMS. Nucl Instr Met Phys Res B 268:989-901.

7. Strzepek KM, Thresher RE, Revill AT, Smith CI, Komugabe AF, Fallon SJ (2014) Preservation effects on the isotopic and elemental composition of skeletal structures in the deep-sea bamboo coral Lepidisis spp. (Isididae). Deep-Sea Res II 99:199-206.

8. Lassey KR, Manning MR, Sparks RJ, Wallace G (1990) Radiocarbon in the sub-tropical convergence east of Tasmania – an interim report. DSIR Phys Sci Rpt #11, DSIR Physical Sciences, Nuclear Sciences Group.

9. Sherwood OA, Thresher RE, Fallon SJ, Davies DM, Trull TW (2009) Tasmanian isidids: isotopic evidence for benthic-pelagic coupling and centennial constancy of oceanographic conditions. Mar Ecol Prog Ser 397:209-218.

10. Komugabe AF, Fallon SJ, Thresher RE, Eggins SM (2014) Modern Tasman Sea surface reservoir ages from deep-sea black corals. Deep-Sea Res II 99:207-212.

11. Tracey DM, Neil H, Marriott P, Andrews AH, Cailliet GM, Sanchez JA (2007) Age, growth and age validation of two genera of deep-sea bamboo corals (family Isididae) in New Zealand waters. Bull Mar Sci 81:393-408.

12. Thresher RE, Fallon S, Townsend AT (2016) A “core-top” screen for trace element proxies of environmental conditions and growth rates in the calcite skeletons of bamboo corals (Isididae). Geochim Cosmochim Acta 193:75-99.

13. Thresher RE, Wilson NC, MacRae CM, Neil H (2010) Temperature effects on the calcite skeletal composition of deep-water gorgonians (Isididae). Geochim Cosmochim Acta, 74:4655-4670.

14. Bindoff NL, Church JA (1992) Warming of the water column in the southwest Pacific Ocean. Nature 357:59–62.

15. Bindoff NL, McDougall TJ (2000) Decadal changes along an Indian Ocean section at 32ºS and their interpretation. J Phys Oceanogr 30:1207–1222.

16. Johnson GC, Orsi AH (1997) Southwest Pacific Ocean Water-mass changes between 1968/69 and 1990/91. J Clim 10:306-316.

17. Gille ST (2002) Warming of the Southern Ocean since the 1950s. Science 295: 1275-1277.

18. Hill TM, Spero HJ, Guilderson T, LaVigne M, Clague D, Macalello S, Jang N (2011) Temperature and vital effect controls on bamboo coral (Isididae) isotope geochemistry: A test of the “lines method”. G3 12 doi:1.0.1029/2010GC003443.

There is NO Competing Interest.

ThresheretalLittleIceAgeSupplementalMaterial.docx

Download PDF

Version 1

posted

You are reading this latest preprint version

Little Ice Age effects on Southern Ocean thermohaline circulation

Status:

Version 1

Abstract

Figures

Introduction

Geochemical proxies and implied AAIW variability

Large-scale drivers of SO dynamics over the last millennium

Termination of LIA impacts and the onset of industrial forcing

Declarations

References

Methods

Additional Declarations

Supplementary Files

Status:

Version 1